Photometry in the dark: time dependent visibility of low intensity light sources

Dirk Poelman,Philippe F Smet

doi:10.1364/oe.18.026293

Abstract

This paper aims at describing the perceived brightness of persistent luminescent materials for emergency signage. In case of emergency, typically, a fully light adapted person is left in the dark, except for the emergency sign. The available photometric models cannot describe visibility of such light source, as they do not consider the slow dark adaptation of the human eye. The model proposed here fully takes into account the shift from photopic to scotopic vision, the related shift in spectral sensitivity and the dark adaptation. The resulting metric is a 'visibility index' and preliminary tests show that it more realistically describes the perceived brightness of persistent luminescent materials than the common photometric standards.

Highlights

For many years, standards for measuring the brightness and color of artificial light sources such as lamps and displays have been well established
The same photometric quantities are used for specifying low level light sources such as persistent luminescent materials, it is realized these are not valid at low light levels [1]
#135414 - $15.00 USD Received 21 Sep 2010; revised 27 Oct 2010; accepted 26 Nov 2010; published 1 Dec 2010 (C) 2010 OSA. Both individual researchers and standards organizations like the CIE (International Commission on Illumination), have undertaken research to develop better photometric practices in low light level conditions. This has led to several models which extend photometry into the mesopic region [7]

Summary

Introduction

Standards for measuring the brightness and color of artificial light sources such as lamps and displays have been well established. A luminance of 0.32 mcd/m2 is taken as the visibility threshold for applications and - by definition - any source with this luminance should look bright, irrespective of its emission spectrum. This is not the case, as the human eye sensitivity shifts from photopic vision at high light levels (corresponding to cone vision) to scotopic vision at low levels (rod vision), both being characterized by a different spectral sensitivity. The present paper aims at a better understanding of human eye behavior in the mesopic and scotopic region, and focuses on persistent luminescent materials as model systems with a high application value [4]

Objectives

Results

Conclusion