Pupil Lumens - PDF Version

   
 
Mercury Utilization
Intro to Induction
NexLume vs HID
Pupil Lumens
Not All Induction Created Equal
PGE Rebate
Tax Incentive

 

 

 


How people see and are psychologically impacted by lighting has been a subject of much study and discussion for years. Describing light as "lumen output" and measuring it as "foot candles" on a work plane have been the traditional ways of describing and defining how much light is required to perform a variety of tasks. However, that is being re-examined based on results of studies on visual performance and the psychological impacts of lighting.
Additionally, the "color rendering index" (CRI) and correlated color temperature (CCT) describe the quality of the light (relating to how true colors appear compared to under a noon north sky on a clear day). As lighting technology evolves into various types and colors, simply measuring the lumens proves not to be fully adequate in predicting how well people can see. An excellent example is the low-sodium lamp, which produces many lumens, but only two colors (yellow and gray); the ability to make out details, beyond shapes of objects, is lost under this light source. Different light sources produce light in different spectral ranges and there is a wide variety of spectral output available in fluorescent lamps.

Vision itself is affected by many factors, from light intensity, distribution, color, and contrast, as well as reflections, glare, air quality, motion of subjects and viewers, and more. Our eyes use different parts to see in bright light and low light conditions. The eye contains cones and rods, which were thought to work in opposite conditions. Cones provide color vision and fine detail (photopic) in bright light and rods take over in dim light (scotopic). In bright light our pupils contract allowing more detail to be perceived, while depth of field and perceived brightness also increase. In low light our eyes dilate to allow more light in.

Light meters and recommended light levels for tasks have traditionally been calibrated for daytime viewing, and general interior lighting, based on the photopic response. However, studies are indicating that the scotopic vision is more involved in interior lighting than thought, and affects pupil size. At recent conferences, some presenters encouraged designers to specify the photopic/scotopic (P/S) ratio of lamps when selecting them in order to get better design, efficiency, and better vision for occupants.

Sam Berman, formerly with the Lighting Systems Research Group at Lawrence Berkeley Laboratory and a major supporter of the importance of the P/S ratio in lighting selection, developed a conversion factor that applies the P/S ratio to lumen output of various light sources, and then expresses the effective lumens the eye will perceive for vision based on the size of the pupil and the effect on vision (see Table 1, pg.2). Some lamps, like low-pressure sodium, lose most of their output using this method, while others like high-quality fluorescent lamps gain substantially.

Induction lamps are basically fluorescents without electrodes. They are equivalent in every way to high-quality fluorescent lamps with a high CRI and a wide range of color temperatures. An additional feature of removing the electrodes in the induction lamp is the extremely long lamp life. Most manufactures of induction systems rate their lamp life at 100,000. Berman's table suggests that, while the T-8 4100 lamp has rated lumens of 90 per watt, the pupil (effective) lumens are actually 145 per watt. The highest value is the high output T-5 5000k and the AES 5000k @ 190. If contrast and distribution are controlled, this suggests that fewer watts are needed to provide good vision than rated lumen output would suggest, meaning energy savings will result.

Table 1. Conversion factors for lumens to pupil lumens Correction factors applied to conventional values of lumens per watt yield a value for pupil lumens per watt, which is a measure of how effectively the eye sees the light that is emitted. The pupil is more receptive to light at the blue end of the spectrum.


   
Light source
Conventional lumens per watt
Correction factor (P/S ratio)
Pupil lumens per watt
Low-pressue sodium
165
0.38
63
5,000-K HO T5 fluorescent
104
1.83
190
4,100-K T8 fluorescent
90
1.62
145
Clear metal halide
85
1.49
126
5,000-K AES induction lamp
95
1.83
190
5,000-K pure tri-phosphor fluorescent
70
1.58
111
3,500-K tri-phosphor fluorescent
69
1.24
85
50-watt high-pressure sodium
65
0.76
49
2,900 K-warm white fluorescent
65
0.98
64
Daylight fluorescent
55
1.72
95
35-watt high-pressure sodium
55
0.57
31
5,000-I 90 CRI fluorescent
46
1.7
78
Vitalight fluorescent
46
1.71
79
Deluxe mercury vapor
40
0.86
34
Standard incandescent
15
1.26
19
Tungsten halogen
22
1.32
29


Summary:
Recent studies seem to favor white light for viewing moving objects in low-light conditions, such as spotting a pedestrian, animal, or other moving object off to the side of the roadway at night. Some cities opt to use white light rather than the yellowish light of high-pressure sodium (even though the price is higher) in hopes of reducing accidents. The improved color rendering of white light in retail areas and places where people congregate after dark
make it a popular choice for street lighting in downtown areas.

White light is proving to have advantages for visual performance. Current codes and standards are based on measurements that do not address the impact of pupil lumens, and pupil lumens can be quite different from traditionally measured lumen output of lamps. Studies on the relevance of light spectrum and the mechanics of vision are ongoing, and codes and standards may reflect that in the future.

Most commonly used Induction lamps produce white light and more fixtures are becoming available, but most carry a premium price and have limited applications. The long life of these lamps can substantially reduce maintenance and hazardous recycling costs due to re-lamping. The introduction of the AES Induction System has changed this misconception. AES offers three distinct shapes, a wide range of wattages (15-200) and Kelvin temperatures from 2700k thru 6000k and with their improved amalgams have all the benefits of the popular high output T-5 lamps. AES has opened up the market for retrofitting and fixture design and can replace most compact fluorescent and HID
installations.

For more information as to the applications and implementation of the creative AES induction system, please contact David Stypula @ (408) 375-8785 or dstypula@comcast.net