The light that enters your eyes travels through the lens and cornea and fall upon the retina the retina consists of RODS and CONES. It was the accepted belief in the scientific community that cones were for day vision and rods for night vision, whilst many felt that this was a little to simple, producers of lighting based all of their calculations on visible or photopic light,thanks to the work of Dr Sam Berman and Dr Don Jewett we now have a far greater understanding of the effect of light on the rods and the subsequent effect on our vision.
What they discovered explains why many people in the UK are complaining that since the low energy lights became common in our homes they find it difficult to see,that print seems blurred and that colours don't seem right. The colour thing is a different issue that I will deal with later the blurring is the critical thing that causes great difficulty to so many people especially the over fifties.
What Dr Berman and Dr Jewett were able to show was a very strong link between the perception of brightness in a room and the spectrum of the lighting in fact what they found was that the rods were controlling the opening and closing of the pupil which is the focus, with high levels of scotopic light the pupil would close and sharpen the focus, where the levels of scotopic light were low the pupil would open and vision would become blurred. It had been known for many years that lamps of a high colour temperature gave the appearance of being brighter even when measurement with a light meter showed them to be less bright but it was not understood why this should be until it was realised that higher colour temperature also meant higher scotopic light levels, consequently an increase in the colour temperature has the effect of closing the pupil and sharpening the focus The light that enters your eyes travels through the lens and cornea and fall upon the retina the retina consists of RODS and CONES. It was the accepted belief in the scientific community that cones were for day vision and rods for night vision, whilst many felt that this was a little to simple, producers of lighting based all of their calculations on visible or photopic light, thanks to the work of Dr Sam Berman and Dr Don Jewett we now have a far greater understanding of the effect of light on the rods and the subsequent effect on our vision.
What they discovered explains why many people in the UK are complaining that since the low energy lights became common in our homes they find it difficult to see, that print seems blurred and that colours don't seem right. The colour thing is a different issue that I will deal with later the blurring is the critical thing that causes great difficulty to so many people especially the over fifties.
What Dr Berman and Dr Jewett were able to show was a very strong link between the perception of brightness in a room and the spectrum of the lighting in fact what they found was that the rods were controlling the opening and closing of the pupil which is the focus, with high levels of scotopic light the pupil would close and sharpen the focus, where the levels of scotopic light were low the pupil would open and vision would become blurred. It had been known for many years that lamps of a high colour temperature gave the appearance of being brighter even when measurement with a light meter showed them to be less bright but it was not understood why this should be until it was realised that higher colour temperature also meant higher scotopic light levels, consequently an increase in the colour temperature has the effect of closing the pupil and sharpening the focus,so it became clear that the best light for people would be a high colour temperature and that not only would people be able to see more clearly but that it would be possible to actually reduce the power used and still have a better light.
The next problem is how to calculate what light we need. When compact fluorescent lamps (CFLs) were being pushed onto the population of the UK by massive government subsidies for whatever reason it was decided that these would have the colour temperature 2700K sometimes described as warm white a misnomer as the light is neither warm nor white it is in fact rather yellow and its colour temperature is considerably below that of an incandescent lamp (light bulb) consequently when equivalence tables were produced to guide people as to which size of CFL would replace each of the commonly used incandescent lamps they were based on photopic output taking no consideration consideration of the scotopic disparity.In order to compare the lamps properly it would be required to take into account the amount of scotopic light as well as the photopic fortunately laboratory tests have been carried out on many light sources and the scotopic photopic ratio is common knowledge :
warm white 2700K fluorescent ---------------- 1
incandescent lamp 2850K---------------------- 1.41
cool white 4000K fluorescent----------------- 1.46
daylight 6500K fluorescent---------------------- 2.14
In order to arrive at perceived brightness or what is sometimes called visually effective lumens, we simply multiply the light meter reading (photopic lumens) by the photopic scotopic ratio, so when we looked at our 827 20Watt cfl with its output of 1200 lumens it initially seems to match a 100Watt light bulb with an output of 1165 lumens but when we take into account the scotpic disparity we are now comparing 1200 x 1 = 1200 with 1165 x 1.41 = 1643 a huge difference, no wonder you can't see. Clearly we could have adopted the cool white cfls which are closest to the incandescent lamps that we are used to and the outcome would have been that the vast majority of people would be very happy with their low energy lamps but what if we adopted the daylight 6500K lamps in order to achieve the same light as a 100W light bulb 1643 adjusted lumens we would only need to use a lamp that produced 1643 /2.14 = 768 lumens we can achieve well in excess of this with a 15W 865 cfl. So thats a better light and saving electricity.
