Spectral Sensitivity Curves Research Articles

A comparison of photographic and spectrometric methods to quantify the colours seen by animal eyes

Abstract Quantifying how colours stimulate animal eyes is an essential first step in many areas of biology, from behavioural and visual ecology to neuroethology. Although multiple methods exist, their relative accuracies remain untested, leaving researchers unsure which methods are most accurate and which might be unacceptably inaccurate. Here, I compare measurements from reflectance spectrometry and four camera‐based techniques to ground‐truth spectroradiometric measurements of radiance, using eight diverse visual systems. Because errors may compound through successive calculations, I calculate not only relative quantum catches but also chroma and colour contrasts. I find that filters mimicking photoreceptor spectral sensitivity curves, through either custom fabrication or weighted additive combinations of bandpass filters (‘computational filters’), as well as reflectance spectrometry, are the most accurate techniques, with most R2 > 0.98. Statistical filters trained on the library of reflectance spectra in the multispectral image calibration and analysis (MICA) toolbox lag behind in accuracy, particularly for dark colours, with R2 values dipping as low as 0.23. However, the performance of these filters is vastly improved (all R2 > 0.96) if one (a) uses the full UV–VIS range of camera and animal vision, even for animals lacking UV photoreceptors and (b) expands the library of training spectra to include simulated low‐reflectance spectra. Statistical filters trained on either a 24‐ or 45‐colour chart (an alternative method offered by the MICA toolbox) vary wildly in accuracy, with R2 values ranging from 0.11 to 0.94, and should not be trusted. For those requiring point measurements only, reflectance spectrometry is highly accurate within the constraints described in the main text. Those using photography and desiring the highest levels of accuracy should opt for custom‐fabricated or computational filters. Statistical filters trained on the MICA toolbox's library of natural spectra can be expected to be reasonably accurate for many combinations of visual systems and stimuli, but only if (a) the full UV–VIS spectrum is used and (b) the training library is expanded to include simulated low‐reflectance spectra. Statistical filters trained on a colour chart are highly unreliable and their use should be discontinued.

Spectral sensitivity of retinal photoreceptors of tortricid moths is not tuned to diel activity period.

Leafrollers (Lepidoptera: Tortricidae) are a large family of small moths containing over 10,000 species, many of which are crop pests. Grapholita molesta, Lobesia botrana and Cydia pomonella adults are sexually active before, during and after sunset, respectively. We wanted to determine whether being active at different times of the day and night is associated with differences in their visual system. Spectral sensitivity (SS) was measured with electroretinograms and selective adaptation with green, blue and ultraviolet light. SS curves could be fitted with a triple nomogram template which indicated the existence of three photoreceptor classes peaking at 355, 440 and 525 nm. The retinae showed clear regionalization, with fewer blue receptors dorsally. No differences among species or between sexes were found. Intracellular recordings in C. pomonella also revealed three photoreceptor classes with sensitivities peaking at 355, 440 and 525 nm. The blue photoreceptors showed inhibitory responses in the green part of the spectrum, indicating the presence of a colour-opponent system. Flicker fusion frequency experiments showed that the response speed was similar between sexes and species and fused at around 100 Hz. Our results indicate that the three species have the ancestral insect retinal substrate for a trichromatic colour vision, based upon the UV, blue and green-sensitive photoreceptors, and lack any prominent adaptations related to being active under different light conditions.

Spectral Filter Selection Based on Human Color Vision for Spectral Reflectance Recovery.

Spectral filters are an important part of a multispectral acquisition system, and the selection of suitable filters can improve the spectral recovery accuracy. In this paper, we propose an efficient human color vision-based method to recover spectral reflectance by the optimal filter selection. The original sensitivity curves of the filters are weighted using the LMS cone response function. The area enclosed by the weighted filter spectral sensitivity curves and the coordinate axis is calculated. The area is subtracted before weighting, and the three filters with the smallest reduction in the weighted area are used as the initial filters. The initial filters selected in this way are closest to the sensitivity function of the human visual system. After the three initial filters are combined with the remaining filters one by one, the filter sets are substituted into the spectral recovery model. The best filter sets under L-weighting, M-weighting, and S-weighting are selected according to the custom error score ranking. Finally, the optimal filter set is selected from the three optimal filter sets according to the custom error score ranking. The experimental results demonstrate that the proposed method outperforms existing methods in spectral and colorimetric accuracy, which also has good stability and robustness. This work will be useful for optimizing the spectral sensitivity of a multispectral acquisition system.

Frequency and time domain characteristics of the insect visual system in optic flow

Frequency and time domain properties of the monochromatic, dichromatic, and trichromatic visual system in an insect have been studied. The research method is based on the representation of the optical flow field of the insect visual system in the form of a dynamic link with an input and output. The Gaussian function was used to approximate the spectral curves. The study of the inertial properties of such a link in the time domain was performed by applying signals to its input in the form of a δ-function or a unit function. The steady-state forced oscillations at the output of the investigated dynamic link are determined when a harmonic effect is applied to the input. A technique for constructing spectral sensitivity curves for monochrome, dichromatic, and trichromatic vision of insects in the frequency domain is proposed; it has been established that the spectra exhibit the properties of ultra wideband signals. It has been found that the dynamic link of the insect visual system is capable of amplifying the optical signal over ultraviolet and blue parts of the spectrum in the time and frequency domain. It has been revealed that at a frequency equal to or higher than the frequency of dielectric relaxation, the optical flow field of the insect's visual system does not sense electromagnetic radiation. This phenomenon is most pronounced in ultraviolet and blue parts of the spectrum. It has been established that the optical signal wideband index μ varies from 0.41 to 1.21; thus, the visual system of an insect has ultra wideband properties. It has been shown that the number of periods of light oscillations in the visual system of insects, which has a spectrum in the form of a Gaussian curve, is related to the exponent μ by a simple relationship. New knowledge gained can be used for creation of matrix solid-state photodetectors in ultraviolet and blue parts of the spectrum.

Multiple opsins in a reef-building coral, Acropora millepora

Opsins, light-sensitive G protein-coupled receptors, have been identified in corals but their properties are largely unknown. Here, we identified six opsin genes (acropsins 1–6) from a coral species Acropora millepora, including three novel opsins (acropsins 4–6), and successfully characterized the properties of four out of the six acropsins. Acropsins 1 and 6 exhibited light-dependent cAMP increases in cultured cells, suggesting that the acropsins could light-dependently activate Gs-type G protein like the box jellyfish opsin from the same opsin group. Spectral sensitivity curves having the maximum sensitivities at ~ 472 nm and ~ 476 nm were estimated for acropsins 1 and 6, respectively, based on the light wavelength-dependent cAMP increases in these opsins-expressing cells (heterologous action spectroscopy). Acropsin 2 belonging to the same group as acropsins 1 and 6 did not induce light-dependent cAMP or Ca2+ changes. We then successfully estimated the acropsin 2 spectral sensitivity curve having its maximum value at ~ 471 nm with its chimera mutant which possessed the third cytoplasmic loop of the Gs-coupled jellyfish opsin. Acropsin 4 categorized as another group light-dependently induced intracellular Ca2+ increases but not cAMP changes. Our results uncovered that the Acropora coral possesses multiple opsins coupling two distinct cascades, cyclic nucleotide and Ca2+signaling light-dependently.

Analysis of the Spectral Sensitivity of Luxmeters and Light Sensors of Smartphones in Terms of Their Influence on the Results of Illuminance Measurements—Example Cases

The article presents spectral sensitivity curves of 22 typical luxmeters and 3 smartphones as a result of laboratory measurements. Based on the measurement data, errors in the spectral correction of the considered instruments and devices were calculated. Due to the fact that the value of the measured illuminance is influenced by the spectral characteristics of the radiation, in order to determine the spectral corrections coefficients, 10 light sources with different spectral distributions of radiation were taken into account. Spectral corrections factors were determined using two methods for all luxmeters and smartphones. This paper presents the relationship between the shape of the spectral sensitivity curves of a given luxmeter and the measured values of f′1 errors. Moreover, the limitations related to the use of spectral corrections in environmental measurements with the use of a luxmeter were demonstrated.

Enhanced short-wavelength sensitivity in the blue-tongued skink Tiliqua rugosa.

ABSTRACTDespite lizards using a wide range of colour signals, the limited variation in photoreceptor spectral sensitivities across lizards suggests only weak selection for species-specific, spectral tuning of photoreceptors. Some species, however, have enhanced short-wavelength sensitivity, which probably helps with the detection of signals rich in ultraviolet and short wavelengths. In this study, we examined the visual system of Tiliqua rugosa, which has an ultraviolet/blue tongue, to gain insight into this species' visual ecology. We used electroretinograms, opsin sequencing and immunohistochemical labelling to characterize whole-eye spectral sensitivity and the elements that shape it. Our findings reveal that T. rugosa expresses all five opsins typically found in lizards (SWS1, SWS2, RH1, RH2 and LWS) but possesses greatly enhanced short-wavelength sensitivity compared with other diurnal lizards. This enhanced short-wavelength sensitivity is characterized by a broadening of the spectral sensitivity curve of the eye towards shorter wavelengths while the peak sensitivity of the eye at longer wavelengths (560 nm) remains similar to that of other diurnal lizards. While an increased abundance of SWS1 photoreceptors is thought to mediate elevated ultraviolet sensitivity in a couple of other lizard species, SWS1 photoreceptor abundance remains low in this species. Instead, our findings suggest that short-wavelength sensitivity is driven by multiple factors which include a potentially red-shifted SWS1 photoreceptor and the absence of short-wavelength-absorbing oil droplets. Examining the coincidence of enhanced short-wavelength sensitivity with blue tongues among lizards of this genus will provide further insight into the co-evolution of conspecific signals and whole-eye spectral sensitivity.

A flexible capacitive photoreceptor for the biomimetic retina

Neuromorphic vision sensors have been extremely beneficial in developing energy-efficient intelligent systems for robotics and privacy-preserving security applications. There is a dire need for devices to mimic the retina’s photoreceptors that encode the light illumination into a sequence of spikes to develop such sensors. Herein, we develop a hybrid perovskite-based flexible photoreceptor whose capacitance changes proportionally to the light intensity mimicking the retina’s rod cells, paving the way for developing an efficient artificial retina network. The proposed device constitutes a hybrid nanocomposite of perovskites (methyl-ammonium lead bromide) and the ferroelectric terpolymer (polyvinylidene fluoride trifluoroethylene-chlorofluoroethylene). A metal-insulator-metal type capacitor with the prepared composite exhibits the unique and photosensitive capacitive behavior at various light intensities in the visible light spectrum. The proposed photoreceptor mimics the spectral sensitivity curve of human photopic vision. The hybrid nanocomposite is stable in ambient air for 129 weeks, with no observable degradation of the composite due to the encapsulation of hybrid perovskites in the hydrophobic polymer. The functionality of the proposed photoreceptor to recognize handwritten digits (MNIST) dataset using an unsupervised trained spiking neural network with 72.05% recognition accuracy is demonstrated. This demonstration proves the potential of the proposed sensor for neuromorphic vision applications.

Synthetic RGB photometry of bright stars: definition of the standard photometric system and UCM library of spectrophotometric spectra

ABSTRACT Although the use of RGB photometry has exploded in the last decades due to the advent of high-quality and inexpensive digital cameras equipped with Bayer-like colour filter systems, there is surprisingly no catalogue of bright stars that can be used for calibration purposes. Since due to their excessive brightness, accurate enough spectrophotometric measurements of bright stars typically cannot be performed with modern large telescopes, we have employed historical 13-colour medium-narrow-band photometric data, gathered with quite reliable photomultipliers, to fit the spectrum of 1346 bright stars using stellar atmosphere models. This not only constitutes a useful compilation of bright spectrophotometric standards well spread in the celestial sphere, the UCM library of spectrophotometric spectra, but allows the generation of a catalogue of reference RGB magnitudes, with typical random uncertainties ∼0.01 mag. For that purpose, we have defined a new set of spectral sensitivity curves, computed as the median of 28 sets of empirical sensitivity curves from the literature, that can be used to establish a standard RGB photometric system. Conversions between RGB magnitudes computed with any of these sets of empirical RGB curves and those determined with the new standard photometric system are provided. Even though particular RGB measurements from single cameras are not expected to provide extremely accurate photometric data, the repeatability and multiplicity of observations will allow access to a large amount of exploitable data in many astronomical fields, such as the detailed monitoring of light pollution and its impact on the night sky brightness, or the study of meteors, Solar system bodies, variable stars, and transient objects. In addition, the RGB magnitudes presented here make the sky an accessible and free laboratory for the calibration of the cameras themselves.

Quantitative Analysis of Mixed Pixels in Hyperspectral Image Using Fractal Dimension Technique

In this paper, a novel approach for identification of the components of a mixed pixel along with the quantitative analysis of the determined components of hyperspectral data has been proposed. The overall method is divided into three phases. First, the spectral sensitivity curves (SSCs) of the identified endmembers soil, water and vegetation using N-FINDR algorithm are generated. Next, the percentage of the mixture of the endmembers is computed for calculating the fractal dimension (FD). Afterward, the FD of SSCs is utilized for determining the mathematical model of the obtained curve. In order to verify that the adjacent pixels belong to the class identified using N-FINDR algorithm, two algorithms, i.e., spectral angle mapper and spectral information divergence, are employed for determining the spectral similarity of the mixed pixel with the reference spectrum. Lastly, the quantitative analysis of the SSC of the mixed pixel is performed by using the corresponding calculated FD in the derived mathematical model. The performance of the mathematical model is evaluated on the airborne visible infrared imaging spectrometer image data, and the outcome shows that the proposed framework achieves very promising results with a small number of mixed pixel samples.

Linear and Non-Linear Models for Remotely-Sensed Hyperspectral Image Visualization

The visualization of hyperspectral images still constitutes an open question and may have an important impact on the consequent analysis tasks. The existing techniques fall mainly in the following categories: band selection, PCA-based approaches, linear approaches, approaches based on digital image processing techniques and machine/deep learning methods. In this article, we propose the usage of a linear model for color formation, to emulate the image acquisition process by a digital color camera. We show how the choice of spectral sensitivity curves has an impact on the visualization of hyperspectral images as RGB color images. In addition, we propose a non-linear model based on an artificial neural network. We objectively assess the impact and the intrinsic quality of the hyperspectral image visualization from the point of view of the amount of information and complexity: (i) in order to objectively quantify the amount of information present in the image, we use the color entropy as a metric; (ii) for the evaluation of the complexity of the scene we employ the color fractal dimension, as an indication of detail and texture characteristics of the image. For comparison, we use several state-of-the-art visualization techniques. We present experimental results on visualization using both the linear and non-linear color formation models, in comparison with four other methods and report on the superiority of the proposed non-linear model.

Circadian Potency Spectrum with Extended Exposure to Polychromatic White LED Light under Workplace Conditions.

Electric light has enabled humans to conquer the night, but light exposure at night can disrupt the circadian timing system and is associated with a diverse range of health disorders. To provide adequate lighting for visual tasks without disrupting the human circadian timing system, a precise definition of circadian spectral sensitivity is required. Prior attempts to define the circadian spectral sensitivity curve have used short (≤90-min) monochromatic light exposures in dark-adapted human subjects or in vitro dark-adapted isolated retina or melanopsin. Several lines of evidence suggest that these dark-adapted circadian spectral sensitivity curves, in addition to 430- to 499-nm (blue) wavelength sensitivity, may include transient 400- to 429-nm (violet) and 500- to 560-nm (green) components mediated by cone- and rod-originated extrinsic inputs to intrinsically photosensitive retinal ganglion cells (ipRGCs), which decay over the first 2 h of extended light exposure. To test the hypothesis that the human circadian spectral sensitivity in light-adapted conditions may have a narrower, predominantly blue, sensitivity, we used 12-h continuous exposures of light-adapted healthy human subjects to 6 polychromatic white light-emitting diode (LED) light sources with diverse spectral power distributions at recommended workplace levels of illumination (540 lux) to determine their effect on the area under curve of the overnight (2000–0800 h) salivary melatonin. We derived a narrow steady-state human Circadian Potency spectral sensitivity curve with a peak at 477 nm and a full-width half-maximum of 438 to 493 nm. This light-adapted Circadian Potency spectral sensitivity permits the development of spectrally engineered LED light sources to minimize circadian disruption and address the health risks of light exposure at night in our 24/7 society, by alternating between daytime circadian stimulatory white light spectra and nocturnal circadian protective white light spectra.

Wild hummingbirds discriminate nonspectral colors.

Many animals have the potential to discriminate nonspectral colors. For humans, purple is the clearest example of a nonspectral color. It is perceived when two color cone types in the retina (blue and red) with nonadjacent spectral sensitivity curves are predominantly stimulated. Purple is considered nonspectral because no monochromatic light (such as from a rainbow) can evoke this simultaneous stimulation. Except in primates and bees, few behavioral experiments have directly examined nonspectral color discrimination, and little is known about nonspectral color perception in animals with more than three types of color photoreceptors. Birds have four color cone types (compared to three in humans) and might perceive additional nonspectral colors such as UV+red and UV+green. Can birds discriminate nonspectral colors, and are these colors behaviorally and ecologically relevant? Here, using comprehensive behavioral experiments, we show that wild hummingbirds can discriminate a variety of nonspectral colors. We also show that hummingbirds, relative to humans, likely perceive a greater proportion of natural colors as nonspectral. Our analysis of plumage and plant spectra reveals many colors that would be perceived as nonspectral by birds but not by humans: Birds' extra cone type allows them not just to see UV light but also to discriminate additional nonspectral colors. Our results support the idea that birds can distinguish colors throughout tetrachromatic color space and indicate that nonspectral color perception is vital for signaling and foraging. Since tetrachromacy appears to have evolved early in vertebrates, this capacity for rich nonspectral color perception is likely widespread.

UV reflectance photography of skin: what are you imaging?

Photography can be a powerful tool for researching the skin. Moving outside the visible spectrum and into the ultra violet (UV) presents a unique set of challenges to the skin photographer because of the restrictions imposed by the equipment being used. This article discusses these challenges in relation to camera sensitivity, lens and filter transmission and lighting spectrum, with the aim of demystifying what is actually being captured when imaging skin. In addition to a discussion of existing data on the subject of camera sensor sensitivity, filter transmission and flash spectral analysis, transmission in the UV of a variety of camera lenses using a new method has been carried out. Using the described approach, lens transmission between 280 and 420nm of a range of lenses has been measured. Combining this with camera sensor sensitivity data and filter and light source characteristics, it has been possible to determine an overall, harmonized, spectral sensitivity curve for what is being imaged with a given setup. UV reflectance photography, while a powerful tool, is often misunderstood and misreported as to what is actually being imaged. By combining measurements on camera sensitivity, lens and filter transmission and light source spectra the researcher can more fully understand what is they are actually measuring, thereby enabling better communication with the consumer on what they are seeing and a more complete description for any claims support.

Spectral sensitivity of L2 biotype in the Thrips tabaci cryptic species complex

The onion thrips, Thrips tabaci (Lindeman, 1889), is a cosmopolitan pest of economic importance on a wide range of crops. Despite being one of the most studied thrips species, there is very limited knowledge available about its ability to perceive light. The T. tabaci cryptic species complex consists of a tobacco-associated (T) and two leek-associated (L1, L2) biotypes. We made electroretinogram recordings on the most widespread thelytokous (where unfertilized eggs produce females) T. tabaci L2 biotype and measured attraction to light sources in this biotype as a function of wavelength in behavioural experiments. The spectral sensitivity of the T. tabaci L2 biotype shows a unimodal curve peaking at λmax = 521 nm. Contrary to this spectral sensitivity curve, L2 biotype attraction in an arena is bimodal with local maxima at 368 nm (UV) and 506–520 nm (green) being practically of the same magnitude. Although being similar to the arrhenotokous (where unfertilized eggs produce males) L1 biotype in phototaxis, significant differences regarding photoreceptor cell responses emerged. This study contributes to our understanding of light perception in Thysanoptera as well as to the development of more effective monitoring tools for this economically important pest species.

Comparisons of Scotopic/Photopic Ratios Using 2- and 10-Degree Spectral Sensitivity Curves

Despite the fact that a 2-degree spectral sensitivity curve (SSC) is extensively used in scientific research and relevant applications, the choice between the 10-degree or the 2-degree photopic SSCs in practical applications for the calculation of scotopic/photopic ratios (S/P ratios) depends on actual needs. We examined S/P ratios for more than 300 light sources for correlated colour temperatures (CCTs) from 2000 K to 8000 K and blackbody radiant spectra from 10000 K to 45000 K using 2- and 10-degree SSCs. Results showed that the ratio of the S/P values calculated using the 10-degree and 2-degree SSCs was approximately equal to 0.916. The average mesopic luminance difference increased from 0% to 5.7% at a photopic adaptation luminance from 0.005 to 5 cd/m2. For most practical applications, the mesopic luminance values calculated using these two SSCs were different by several percentage units, yet these differences could be neglected. At extremely high CCTs over 10000 K, the mesopic luminance difference may approximate the maximum value of 16%. This work proposes the conversion coefficients for S/P ratios and the transforming mesopic luminance values calculated for 2- and 10-degree SSC systems. These results may help researchers clarify differences between the S/P ratios calculated using different SSCs.

Self-referenced plasmonic sensor with TiO2 grating on thin Au layer: simulated performance analysis in optical communication band

Dielectric grating–based surface plasmon resonance (SPR) sensors with self-referenced capability are relatively less explored. A self-referenced SPR sensor incorporating titanium oxide (TiO2) grating on thin gold (Au) film atop dielectric substrates is proposed in the optical communication band. The rigorous coupled wave analysis (RCWA) method is used for sensor simulation and evaluation of performance parameters. The finite-element method-based field strength analysis is performed to study the behavior of two plasmon modes (analyte mode and substrate mode) at respective resonance conditions. Further, the influence of various grating variables on sensor performance is analyzed. As a major result, in the optical communication band, an average spectral sensitivity and SPR curve width of 693.88 nm/RIU and 26.03 nm, respectively, are achieved for the optimized values of grating variables. Proposed TiO2 grating on Au film atop SiO2 substrate can provide greater sensitivity than the existing grating-based plasmonic sensors for a broad range of analyte refractive indices.

Spectrosensitometer with wide spectrum, large scale and multistep optical field

A new type of spectrosensitometer has been developed, which is characterized by a wide spectrum range of 340 nm ~ 900 nm and a large exposed area of 202 mm × 90.5 mm with multi-step light intensities on it. The optical density value error of each step on the 18-step wedge with high precision is not greater than 0.01. The film filter evaporated according to the spectral characteristic of the light source can eliminate the secondary spectrum of grating. The automatic control acquisition system is developed by LabVIEW and all-in-one PLC with HMI. In the horizontal direction, the grating displacement sensor is adopted to form the closed-loop control, and the wavelength positioning deviation is less than 0.05 nm. Linear compensation method is adopted in the vertical direction with a height deviation of less than 0.05 mm. The spectrosensitometer automatically measures the optical power per unit area of lights with different wavelengths and light intensities on the step wedge. The shutter controls exposure time. Photosensitive materials are once exposed within the scope of the wide spectrum. After being developed and fixed, the optical density value can be measured by densitometer. The spectral sensitivity curve of a photosensitive material with a certain optical density value can be drawn according to the national standard (GB10557-89).

New spectral templates for rhodopsin and porphyropsin visual pigments

A four-parameter model of spectral sensitivity curves was developed. Empirical equations were designed for A1- and A2-based visual pigments with the main ?-band maximum absorptions (?max) from 350 nm, near the ultraviolet, up to 635 nm in the far-red part of the spectrum. Subtraction of the ?-band from the full absorbance spectrum left a ??-band? described by a ?max-dependent Gaussian equation. Compatibility of our templates with A1- and A2-based spectra was tested on the electroretinographic (ERG-derived) scotopic action spectra recorded in dogfish shark, eel, Prussian carp and perch. To more precisely estimate the accuracy of our model, we compared it with widely used templates for visual pigments. There was almost no difference between the tested models in fitting the above-mentioned spectral data. One of the advantages of our model is that in the fitting of spectral sensitivity data it uses non-transformed wavelengths and the shape of the curve remains the same for a broad range of ?max values. Compared to multiparameter templates of other authors, our model was designed with fewer (four) parameters, which we believe can bring us closer to understanding the true nature of the absorption curve.

Space grating optical structure of the retina and RGB-color vision.

Diffraction of light at the spatial cellular phase grating outer nuclear layer of the retina could produce Fresnel near-field interferences in three RGB diffraction orders accessible to photoreceptors (cones/rods). At perpendicular light incidence the wavelengths of the RGB diffraction orders in photopic vision-a fundamental R-wave with two G+B-harmonics-correspond to the peak wavelengths of the spectral brightness sensitivity curves of the cones at 559nmR, 537nmG, and 447nmB. In scotopic vision the R+G diffraction orders optically fuse at 512nm, the peak value of the rod's spectral brightness sensitivity curve. The diffractive-optical transmission system with sender (resonator), space waves, and receiver antennae converts the spectral light components involved in imaging into RGB space. The colors seen at objects are diffractive-optical products in the eye, as the German philosopher A. Schopenhauer predicted. They are second related to the overall illumination in object space. The RGB transmission system is the missing link optically managing the spectral tuning of the RGB photopigments.