Getting closer to more accurate insight This article is going to reflect vision and insight. Two words yet full of meanings which can inspire us and bring us to awe. A lot of beautiful thoughts could be written but we will narrow down our attention to light which enables us to see. Through sight we receive majority of information about reality around us but also it is able to stimulate creativity, sparking inspiration and urge our expression, communicate beauty. What light does our sight need? What kind of light will serve best particular application? We may feel lost in complexity of the topic. However, systematic approach applied to achieve deeper understanding of the mechanisms of the visioncan bring better clarity compared to perceived subjectivity. Sight itself has been object of exploration for ages. However rigorous scientific, in particular physical quantitative examination can be dated to the previous century. Simple reasons can be identified for the late research uptake: (i) high variability of observer-to-observer performance, (ii) lack of detectors suitable for measurement of light, (iii) bigger attraction of society and physicists to electricity and other disciplines. The last two hinderers have been overcome while the first one has affects modern photometry and colorimetry. Let’s focus on photometry. Quantifying sensitivity of human eye, i.e. how many photons the eye detects, is cornerstone for photometry and colorimetry. Sensitivity of so called photopic regime, i.e. at illumination equal to sunny day levels where colour perception and discrimination is possible, was first done in 1920s. The result was luminosity function, V(λ), determining the lumen per (radiation) Watt conversion coefficient. In other words how bright the optical, monochromatic radiation at particular wavelength appears. Of course the estimated function is based on the averaged results of the participated observers. This sensitivity function was revised and released in the set CIE 1931 Color Matching Functions in 1931 as function, and is shown in Figure 1. Maximum value occurs at V(λ = 555 nm) = 683 lm/W.
Thorough decades a lot of mismatch especially between colorimetric predictions and visual experimental observations have been encountered leading to conclusion that CIE 1931 Color Matching Functions set is inaccurate and shall be revised. Despite several proposals this has not happened but CIE set TC 1-36 committee to come-up with new set of functions. Draft functions though not the final ones yet are already available including the Luminosity function. Both functions are shown in Figure 2.
Figure 2: CIE 1931 Luminosity function (blue) and CIE TC 1-36 drafted Luminosity function (red).
It can be observed that the new function shows higher sensitivity in the blue and the amber-red part of the spectrum. Therefore light sources with strong blue content – as standard white LED (blue LED pump and yellow phosphor), by the new measure shall be measured with higher light output. A comparison of normalized lumen output for the current and proposed Luminosity function are given in the Table 1 calculated by the following equation
Table 1: Calculated lumen output for normalized spectral power distribution of standard light sources, compact fluorescent light (CFL) and various white LEDs.
where Φe stands for spectral radiant flux of the light source and VX(λ) stands for Luminosity function; both CIE 1931 and TC 1-36.
Although the drafted Luminosity function may not be the final one it shall not deviate much from the examined standard. Polychromatic light sources shall be given higher normalized lumen values and the differences due to chromaticity or spectral shape shall be up to 2 percent.
Figure 1: CIE 1931 Luminosity function for standard observer.
www.ilumtech.eu
The table contains various light sources: warm and cold, incandescent, fluorescent and LED based. In all cases the normalized luminous flux was higher for the draft Luminosity function by approximately 5%. Difference between maximum and minimum value was 0.8% in the listed cases. The maximum difference found for a longer list was 3.3% for metal vapor lamps.
info@ilumtech.eu