Phosphor Persistence Research Articles

Gamma Scientific Sets High Standards for Light Measurement

Gamma Scientific Sets High Standards for Light Measurement

Effect of varying CRT refresh rate on the measurement of temporal summation.

To quantify the effect of cathode-tube-ray (CRT) monitor refresh rate on the measurement of the upper limit of complete temporal summation (critical duration) in the peripheral visual field of healthy observers. Contrast thresholds were measured for seven achromatic spot stimuli (diameter 0.48°) of varying duration (nominal values: 10-200 ms) at an eccentricity of 8.8° along the 45°, 135°, 225° and 315° meridians of the visual field in three healthy, psychophysically experienced observers. Stimuli were presented on a CRT display with a refresh rate of 60 and 160 Hz. Contrast thresholds were expressed as contrast energy with stimulus durations being estimated using (1) the sum-of-frames (SOF) method and (2) Bridgeman's method incorporating measurements of phosphor persistence. Estimates of the critical duration were produced using iterative two-phase regression analysis. With stimulus duration expressed as SOF equivalent the critical duration was, on average, 10.6 ms longer with a refresh rate of 60 Hz (mean 45.7 ms, S.D. 10.1 ms) relative to 160 Hz (35.1 ms, S.D. 7.6 ms). When the Bridgeman method was used, minimal differences (1.8 ms) in critical duration values between the two refresh rates (60 Hz: 33.0 ms, S.D. 9.4 ms; 160 Hz: 31.2 ms, S.D. 7.0 ms) were observed. Identical trends were observed in all three subjects. Psychophysical measurements of temporal summation are independent of variations in CRT refresh rate when the Bridgeman method, incorporating measured values of phosphor persistence, is used to estimate stimulus duration. This has significant implications for the specification of stimulus duration in psychophysical studies of vision employing conventional display monitors.

Effects of Phosphor Persistence on High-Speed Imaging of Transient Luminous Events

High-speed intensified cameras are commonly used to observe and study the transient luminous events known as sprite halos and sprite streamers occurring in the Earth’s upper atmosphere in association with thunderstorm activity. In such observations, the phosphor persistence in the image intensifier, depending on its characteristic decay time, might lead to a significant distortion of the optical signals recorded by those cameras. In this paper, we analyze the observational data obtained using different camera systems to discuss the effects of phosphor persistence on high-speed video observations of sprites, and introduce a deconvolution technique to effectively reduce such effects. The discussed technique could also be used to enhance the high-speed images of other transient optical phenomena in the case when the phosphor persistence has a characteristic decay time that is comparable with the temporal resolution of the cameras required to resolve the phenomena.

High‐speed intensified video recordings of sprites and elves over the western Mediterranean Sea during winter thunderstorms

We report the first intensified high‐speed video images of elves, sprites, and halos observed in Europe. All the events corresponded to winter season thunderstorms over the Mediterranean Sea. The observations comprise many elves generated by both cloud‐to‐ground lightning current polarities. In 8 of the 14 sprite observations we observed an elve previous to the sprite. In three cases we observed also an elve quickly followed by a halo and a sprite. In several observations we observed lightning light before the mesospheric transient luminous event. We present a case where the lightning from cloud tops was visible during the entire event. Thanks to the high‐speed videos and their resolution and low intensifier phosphor persistence we analyzed the timing distribution of the development phase of sprite elements, the persisting luminosity phase, and the total duration. Finally, we summarize one particular observation where a streamer collides and bounces with a previous formed column; it may be a new phenomenon of collision between an existing column body that interacts with a later streamer point‐like tip which is not merged and attached.

Spatiotopic Transfer of Visual-Form Adaptation across Saccadic Eye Movements

Although conscious perception is smooth and continuous, the input to the visual system is a series of short, discrete fixations interleaved with rapid shifts of the eye. One possible explanation for visual stability is that internal maps of objects and their visual properties are remapped around the time of saccades, but numerous studies have demonstrated that visual patterns are not combined across saccades. Here, we report that visual-form aftereffects transfer across separate fixations when adaptor and test are presented in the same spatial position. The magnitude of the transsaccadic adaptation increased with stimulus complexity, suggesting a progressive construction of spatiotopic receptive fields along the visual-form pathway. These results demonstrate that basic shape information is combined across saccades, allowing for predictive and consistent information from the past to be incorporated into each new fixation.

Vision with a scanning laser display: comparison of flicker sensitivity to a CRT

Laser scanning displays allow for large depth-of-focus, reduced optical aberrations, and high luminance. However, scanned laser light images have no phosphor persistence. Psychophysical tests with a 3-color laser display showed that flicker thresholds were nearly equal with a laser display relative to the CRT when both were matched in luminance, resolution, frame rate, and similar chromaticity. The threshold ratio was 1.14 (range 0.7–1.6). Differences in flicker sensitivity between displays were significantly related to the absolute energy at the cornea irrespective of the subjective luminance match. Flicker sensitivity was not enhanced by a laser scanning display without image persistence.

Luminance artifacts of cathode-ray tube displays for vision research.

Phosphor persistence, video bandwidth, DC restoration and high-voltage regulation affect the appearance of images presented on cathode-ray tubes (CRTs), potentially resulting in differences between nominal and actual stimuli. We illustrate these effects by measuring physical parameters of horizontal and vertical static and counter-phase flickering gratings, and we illustrate problems for vision research by measuring contrast sensitivity to these gratings. We also measured the extent to which calibration protocols actually result in the monitor being calibrated over its entire area regardless of image size. The results of our physical measurements indicate substantial differences between gratings that nominally differ only as to orientation. Consistent with these differences, our psychophysical measurements indicate different sensitivities when the bars of the gratings are parallel or orthogonal to raster lines, regardless of the retinal orientation of the gratings. The results of our calibration check show that only a small region around the target area of calibration can be regarded as effectively linearized, and only if the size of the test image used during the check is similar to the size of the calibration patch. Overall, our results indicate potentially severe problems with the use of CRTs in vision research, and we discuss some published results that are likely to have been affected by these problems.

A fast phosphor imaging diagnostic for two-dimensional plasma fluctuation measurements

A plasma imaging diagnostic is being developed using a fast time response ZnO:Zn phosphor disk to image plasma density fluctuations in a two-dimensional (2D) region. The plasma sensor consists of an 8.9 cm diameter phosphor coated aluminum disk that is inserted into the plasma and is excited by incident electrons resulting in a cathodoluminescent emission image. The phosphor light distribution is then interpreted as plasma density fluctuations using sheath theory and the phosphor response function. The local luminance S of fluctuating phosphor light is dependent on the incident electron energy Ee and current density through the equation S(r,t)=e∫R(Ee)(v⋅n̂)fe(r,v,t)d3v, where R(Ee) is the energy dependent response function of the phosphor. The phosphor persistence time of 1–10 μs (emulsion dependent) combined with fast intensified charge coupled device camera shutter speeds enables the imaging of plasma fluctuations on microsecond range time scales. Broadband turbulent phosphor light fluctuations (f⩽500 kHz) have been measured with photodiode detectors and compared with Langmuir probe data. The 2D phosphor images show density profiles consistent with radial Langmuir probe measurements. Temporal and spatial resolution of the phosphor diagnostic enables 2D imaging of plasma turbulence and coherent modes.

ＤＣ形ＰＤＰを用いた時分割２眼立体表示法におけるクロストーク妨害とその改善

We previously developed a field-sequential stereoscopic display system with a 42-inch diagonal HDTV DC-type plasma display panel (PDP). This system uses an LCD shutter stereoscopic eyeglass, synchronized with the left and right pictures on the display by infrared signals. Stereoscopic images were displayed on the PDP using the proposed light emission scheme and gray-scale signal processing. However, it was found that cross-talk impairment was incurred due to leakage between light for the left and right images. We calculated the amount of cross-talk as a function of phosphor persistence and decay time and estimated the desirable decay time for a field-sequential stereoscopic display system. This enabled us to develop a signal processing method for reducing the cross-talk impairment, and a subjective evaluation confirms the effectiveness of the method.

The effect of phosphor persistence on image quality in digital x-ray scanning systems.

A digital x-ray scanning system offers several advantages over conventional film-screen systems. However, there are sources of image degradation resulting from the scanning motion, such as motion blur due to the temporal response of the phosphor. This mechanism produces an asymmetrical blur, requiring the use of the complex optical transfer function (OTF) rather than the normal modulation transfer function (MTF) for correct characterization of image resolution. The luminescence response of eight phosphors was measured under pulsed x-ray excitation. A weighted exponential model was used to represent the primary luminescence. The dominant luminescence life-times ranged from 2.7 microseconds for Gd2O2S:Pr to 558 microseconds for Gd2O2S:Tb. The long term response was also measured, monitoring significant increases in a slow form of luminescence known as afterglow. Afterglow was modeled by an inverse power law equation. Afterglow was found to be strong in two of the phosphors studied (ZnCdS:Ag and YTaO4). In selecting a phosphor for a scanning system, it must satisfy several criteria, including a fast temporal response. Thus, a phosphor like Gd2O2S:Tb, which has a slow luminescence, but otherwise excellent imaging properties, may not be as useful as a more rapid phosphor like CsI:Tl.

Eye-movement-contingent display changes are not compromised by flicker and phosphor persistence

The eye-movement-contingent display change technique has been used in a large number of studies to control the visibility of text when eye movements are monitored. Recently, the validity of the method has been questioned. O’Regan (1990) argued that technical imperfections in the implementation of the display change, rather than manipulation of visible text, account for some of the reported effects. This view was tested in the present study by manipulating characteristics of the display change and the linguistic task. Consistent with prior results, reading was hampered when eye-movement-contingent display changes were used to mask segments of text prior to their fixation. However, these costs were unrelated to the quality of the implemented display change and appear to have derived solely from the masking of useful text.

P31 phosphor persistence at photopic mean luminance level.

P31 phosphor screens are frequently used for short-term presentation of dot and grating patterns, but experimental data obtained with this technique have been criticized because of possible parasitic effects of phosphor persistence on subjects visual performance. Recently, this issue provoked a controversial discussion in Vision Research (Groner et al., 1993; Westheimer, 1993, 1994; Irwin, 1994; Di Lollo et al., 1994) which was concerned with persistence effects of P31 screens for dot patterns. Supplementing this discussion, the present work deals with the effects of different types of patterns (dot pattern vs. gratings) and background mean-luminance levels (scotopic vs. phototopic) on phosphor persistence. Physical measurements of P31 persistence occurring with grating patterns of a mean luminance of 20 cd m-2 (i.e. photopic range) were obtained by using an extremely linear photometer with high temporal resolution. Under this photopic condition, the measurements demonstrate a fast decay of residual grating contrast to 1.4% of its original value within 50 ms after pattern offset. This phosphor behavior must be considered when designing an experiment with a P31 screen though it certainly embodies no problems in many applications.

Phosphor persistence of oscilloscopic displays: a comparison of four phosphors.

The period for which phosphor decay remains visible after stimulus offset was assessed for four phosphors commonly used in psychophysical experiments: P4, P15, P31, and P46. Stimuli were displayed behind closed shutters which opened at various intervals after stimulus offset. Thus, the observers' responses were based solely on the visibility of phosphor persistence. We varied viewing conditions (dark-adapted vs. veiling light), type of task (detection vs. identification), and intensity of the stimuli. No detectable persistence was ever produced by the P15 phosphor. In contrast, the P31 phosphor remained visible for several hundred ms. even with a veiling light. The P4 and P46 phosphors produced persistence of intermediate durations. It is concluded that P15 is the phosphor of choice for visual experiments.

A note on luminance calibration of raster-scan cathode-ray tubes: temporal resolution, ripple, and accuracy

Accurate luminance calibration is an important issue for stimuli in vision research. Raster-scan cathode-ray tubes present a rapidly scanning spot with a luminance of the order of 10,000 cd m-2 (the actual value depends on video timing and phosphor persistence). With little spatial integration this may result in overloading of the front stage of a photometer. More importantly, even if averaged over hundreds of milliseconds, the pulsed nature of the luminance signal will markedly reduce accuracy of the luminance measurement. A frame-synchronised photometer is described to increase measurement accuracy whilst still rapidly acquiring the result.

Visual dissociation of digitized photographs

Visual dissociation occurs when a visual marker from one display in a rapidly presented sequence (e.g., 9 item/sec) is perceived as having occurred in a temporally adjacent display. Three experiments that evaluated the application of computer graphics technology to study this phenomenon with digitized color photographs are reported. The primary concern was that visible phosphor persistence might artificially increase the frequency of integration errors. In Experiment 1, visible phosphor persistence was assessed using a multiple-repetition shutter test to determine which stimulus conditions did not yield reportable persistence. On the basis of these results, visual dissociation performance when the same color photographs were presented using mechanical 16-mm projection (as in previous research) and when they were presented on a computer monitor were compared in Experiments 2 and 3. The results supported computer application, in that computer presentation yielded the same pattern of errors and accuracy levels as did research using mechanical projection.

Phosphor persistence in oscilloscopic displays: its luminance and visibility

Phosphor persistence and its potentially confounding effects in visual experiments were discussed recently in these pages (Groner, Groner, Miiller, Bischof & Di Lollo, 1993). In that report, we showed that phosphor persistence can remain visible for extended periods (in the order of hundreds of msec) on oscilloscopic screens coated with P31 but not with P15 phosphor. In a dissenting research note, Westheimer (1993) dismisses our evidence as stemming from “very indirect exper- iments” and concludes that the persistence of P31 phos- phor, as measured by a photometer, decays within about 2 msec. This 2-msec estimate could be easily taken as applying equally to photometers and to human observ- ers. To obviate this misinterpretation, the generality of Westheimer’s conclusion needs to be qualified. A common reason for measuring the temporal course of phosphor decay is to provide engineering specifica- tions for display oscilloscopes (e.g. Bell, 1970). For this purpose, the measuring instrument of choice is a photo- meter, not the human eye. As pointedly noted by Westheimer (1993), human eyes would supply only indirect estimates. We agree: instead of the direct lumi- nance readings provided by a photometer, visual re- sponses yield only

On the measurement of phosphor persistence in oscilloscopic displays

On the measurement of phosphor persistence in oscilloscopic displays

On the confounding effects of phosphor persistence in oscilloscopic displays

Phosphor persistence has been a source of confounding in studies of temporal integration in vision. We examined the confounding by assessing the effects of the persistence of two commonly-used phosphors (P15 and P31) on performance of a temporal-integration task. In one experiment we eliminated the visibility of phosphor persistence by closing two mechanical shutters upon display termination. In a second experiment we estimated the duration of phosphor persistence by displaying the image behind closed shutters which opened upon display termination. No detectable persistence was every produced by P15 phosphor. By contrast, P31 phosphor produced persistence that lasted several hundred milliseconds even when a veiling light was projected on the screen. We ascribe the earlier instances of confounding to inadequate interpretation of the technical data on phosphor decay.

Restoration of moving binary images degraded owing to phosphor persistence

The degraded images of dynamic objects obtained by using a phosphor-based electro-optical display are analyzed in terms of dynamic modulation transfer function (DMTF) and temporal characteristics of the display system. The direct correspondence between the DMTF and image smear is used in developing real-time techniques for the restoration of degraded images.

Effects of phosphor persistence on perception and the control of eye movements.

When a rapid eye movement (saccade) is made across material displayed on cathode ray tube monitors with short-persistence phosphors, various perceptual phenomena occur. The phenomena do not occur when the monitor has a long-persistence phosphor. These phenomena were observed for certain spatial arrays, their possible physiological basis noted, and their effect on the control of eye movements examined. When the display consisted simply of two dots, and a saccade was made from one to the other, a transient ghost image was seen just beyond the destination target. When the display consisted of vertical lines, tilting and displacement of the lines occurred. The phenomena were more intrusive for the latter display and there was a significant increase in the number of corrective saccades. These results are interpreted in terms of the effects of fluctuating illumination (and hence phosphor persistence) on saccadic suppression.