The LED technology in the light market has caused quite some questions and need for clarifications. Our experience is that the main issues which are being discussed is the measurement of lux levels when using LED technology and the use of a S/P factor to calculate Pupil lumen.


Light measurement

In Luminell we are doing light measurement as it always has been done, by using photometric lux metres which is available on the market. The photometric lux metres are calibrated according to the photopic visual mode, which is the "daylight mode" of the eye. In photopic mode the eye is most sensitive to apx 555nm. which the photometric lux metres also are calibrated to be.


A standard photometric Lux meter,  is used as reference for all Luminell simulations and specifications. An example of such Lux meter is the Amprobe LM-120, please see picture below.



S/P factor and pupil lumen

A relevant question in this regards lux levels, is if it's recommended to use a S/P factor to calculate Pupil Lumen. This because the light from the LED light sources looks much brighter then i.e light from HPS.


The S/P factor is a correction factor to calculate pupil lumen from photopic lumen in mesopic light conditions. Mesopic light conditions are when it's dark, but still there's enough ambient light to see colors, typically night driving conditions, with luminance levels from apx 0,001 cd/m2 to apx 10 cd/m2. These are also the typical light conditions that will be found on the brigde on a vessel by night, but not on a well illuminated working deck. Then the eye will be in a photopic vision regime, please see figure below.




In mesopic regime the eye is very sensitive for light, since both the photoreceptors, rods and cones, in the eyes are active. As described earlier the cones are most sensitive at 555nm (683 W/lm), and the cones are active from apx 0,001 cd/m2 and above.


The rods are much more light sensitive, and have their highest sensitivity at 507 nm (1.700 lm/W). The rods are active at low light conditions starting at 100 cd/m2 and below.




In the mesopic vision regime, both rods and cones are active, which means that the eye is more sensitive to light than a photometric lux meter is actually measuring. Then the impression is that what is seen is not the same as what is being measured. This is espesially evident when replacing HPS light sources with LED light sources, since the LED light sources has far better light quality.


Onboard vessels and rigs, there are quite some aspects to take into account when it comes to illumination. Important aspects is the crew both on the bridge and on working deck. The crew working at night on the brigde, are probably in a mesopic vision regime since the environment is dark, but there's enough ambient light to keep the cones active so that they see color. CIE (International Commission on Illumination) is working on an publication for mesopic photometry, please find more information here.


The crew on deck are working in the illuminated area, which is illuminated with typically 100 lux and above measured 0,8 meter above deck. The eyes are then more in the photopic vision regime, with the rods less active or not active at all. Since the eye is in a photopic vision regime, Luminell doesn't use or recommend to use the S/P factor in order to reduce lux levels on deck, but we measure the lux levels with a normal photometric luxmeter.


Regarding illumination on deck it is also important to keep in mind the positive effect good illumination has on the day rythm of the body and the ability to stay sharp and alert in very demanding working situations.


For the information the S/P factor for the RLX series is 1,79.