Question:  I'm interested in LED streetlight technology and have read about scotopic light measurements.  What can you tell me about that term?

Answer:  LED street lights seem to be the new rage-everyone is interested and there is a plethora of products to choose from as well as more than enough salespeople to keep us all busy.  According to the most recent Department of Energy independent testing less than half measure up to the manufacturer’s claims.  One important advantage they tout is the significantly higher output of pupil or scotopic lumens.  LED Street light manufacturers refer to Scotopic light levels in relation to their energy savings. They claim a 70-watt LED lamp (6900 lumens) can take the place of a 250 watt HPS fixture (25,000 lumens) because it emits more than twice the Scotopic light levels as compared to a HPS lamp source. 

So, what is Scotopic light and how does it relate to night vision and street lighting?  The human eye has two types of light receptor cells commonly referred to as rods for scotopic (night) vision and cones for photopic (daylight) vision.  The cones are generally concentrated in the center of the retina around the fovea (focal point of the retina) and the cones are mostly in the surrounding area.  Consequently when looking directly at any object you are using mostly the cones.  They are able to see color whereas the rods see black and white or shades of grey.  In order to see in the dark, the rods require visual purple (rhodopsin) which is produced when the eye is exposed to low light conditions. Visual purple is not produced instantaneously as we all know from walking out of a brightly lit area into the dark.  In fact, it takes as much as 30-minutes for the eye to fully adapt to darkness.  But even after fully adapting we still see poorly when looking directly at an object at low light levels because of the limited number of rod cells around the fovea.  It is also important to understand the eye’s adaptation to darkness is somewhat relative to the light level. The brighter the scene the less visual purple produced. The combination of the slow production of visual purple combined with the eyes adaptive response based on the light levels is an important consideration in street lighting.

It is also important to understand that the eye is best able to discern details when the contrast ratio (max to min ratio) is below 10 to one.  What this means is if the street lights are too bright creating a glare and hot spots that the details are washed out and your ability to discern contrast, depth or detail is reduced. Further these light bombs, as I call them, restrict the production of visual purple making shadowed areas darker. Add to this the fact that as we age the cornea and the fluid in our eyes becomes less clear and light is scattered in the eye.  This makes it more difficult to see in glare situations.  A good example of this is a low sun shining on the windshield of your car.  The optimum light for night vision is clearly a uniform light level at a level that works well with the human eyes adaptation to the dark and without glare.

The cones of the eye are not all the same.  There are three types of cones each with sensitivity to different spectrums of light. Consequently if a light source produces a limited spectrum of light some of the cones are not receiving light in a spectrum they are sensitive to.  The cones are fully effective in the photopic range above 34 cm², (candela per meter squared), and are more sensitive to light in the 550 nanometer (Yellow) range in the photopic range.  However as light levels drop the eye becomes increasingly sensitive to lower wave length light in the blue green spectrum.  This means that the eye responds well to the yellow HPS source provided it is bright enough.  However, because of the very poor CRI of HPS light some of the cones are less effective.  A light source with good color rendering characteristics helps improve night vision in the lower light levels typical of street lighting and at the same lumen levels a white light source will appear 30% brighter than a HPS source.

Scotopic vision occurs at light levels below 3.4 cm² when the cone cells are ineffective and the eye depends on the rods. The rod cells are more sensitive to light in the 507 nanometer blue green range.  It is this light spectrum referred to as scotopic or sometimes pupil lumens.  However the minimum light level in the IESNA guidelines is 3cm².  In fact most street lighting falls between scotopic and photopic or the mesopic range.  Consequently most all street lighting is at levels above the scotopic range and thus the importance of scotopic light is greatly diminished.  At the same time scotopic light is very helpful with aiding the peripheral vision at night as that area of the eye is primarily filled by the rods. 

One other affect of light in the blue green spectrum is the response of the pupil.  Blue green wave length light causes a stronger reaction of the pupil.  Consequently when the blue green spectrum is present the pupil gets smaller than the same amount of lumens from a HPS source. When the pupil is smaller the eye has improved focus and a greater depth of field.  This is the same as increasing the F stop of a camera.  At the same time the smaller pupil opening does let in less light. This would argue that for lighting in the mesopic range a light source that has good color rendering characteristics benefiting both the rods and cones.

While scotopic light is important at very low light levels, given the recommended levels of street lighting by the IESNA RP-8 guidelines, it is not as important as many manufacturers would have you believe.  LED lights are a white light source with typically a color rendition index around 70.  The fact is a white light source can be approximately 30% less bright but will appear just as bright as a HPS source. LED lights also tend to be directional and thereby focus more footcandles on the ground gaining perhaps another ten percent advantage.  A well designed LED light can provide more uniform distribution of light with improved average to min ratios and less hot spots. So while they do enjoy some advantages over HPS sources levels are not as important as the manufacturers would have you believe and you can not effectively replace a HPS source with one that produces one fourth the lumens. At the same time be cautious of glare which can negate these advantages. 

If approached by a salesman

1.      Ask to see the LM79 and LM80 test results,

2.       Ask if it has been IP (Ingress Protection) tested. 

3.      Ask what the “T” junction temperature is after it has reached a stable operating temperature and at what ambient temperature the readings were taken. Generally we want to see temperatures below 125 degrees centigrade.

4.      Ask what the operating forward current is.  Optimum is 350 milliamps at a max T junction temperature of 125 degrees centigrade.  If the T junction temperature is lower than the LED can stand higher forward currents. 

5.      Ask about any testing for color shift and rate of lumen loss over the projected life of the LED. 

6.      Ask if the driver life expectancy is equal or greater than the L70 rated life of the LED.  L70 is the point at which the LED will put out 70% of its original lumens.

7.      How long has the company been in business and will they be around to stand behind their warranty.

8.      How much of a warranty will they provide?

 

If they can provide these answers from an independent testing lab they are worth investigating further.  We are finally seeing some fixtures that are reasonably well designed and reaching system efficacy rates of 70+ lumens per watt (HPS is around 55 lumens per watt).  They are not cheap- ranging in price from $400 to over $1000 per fixture depending on the number of LEDs.  It is reasonable to assume with the better LED streetlights available today you can expect to achieve a 40% to 50% savings in energy.  The technology is improving dramatically each year and these numbers are expected to exceed 100 lumens per watt within the next year. The best way to stay abreast of the development of this exciting technology and to get honest facts is to log onto the DOE solid state lighting web site. Home page:  www.ssl.energy.gov For test results www.ssl.energy.gov/caliper.html

Authored by: George Woodbury/Republic ITS