Spot the Omission . .

Started 6 months ago | Discussions thread
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xpatUSA
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Spot the Omission . .
6 months ago

This topic is prompted the 'Spot the Error'thread which, unfortunately, devolved into a welter of rash statements and misunderstandings, some of them mine

This graph was presented by one of our Worthy Members:

image courtesy of Hamamatsu

The image was presented to justify the use of illuminance to show the performance of a photodiode as current versus illuminance, as opposed to the more common use of current (per Watt) versus irradiance.

My claim is that the graph as presented, although accurate, is incomplete - unlike a similarly shaped graph of current (per Watt) versus irradiance. A claim that I tried to make in the preceding thread but not clearly enough, it seemed.

Taking the example of 100 lux in the above, the diode outputs 0.5uA. So, how many Watts/sq.meter is 100 lux (lumens/sq.meter)?

Or, to keep it simple, how many Watts is 100 lumens?

You will find that this question can not be answered unless you also know the wavelength of the light which, in the Hamamatsu document is not stated. However, in a lighting handbook I have, it gives some values for lumens per Watt at various wavelengths. At 555nm it is, of course, 683 lm/W. At 650nm, aka 'red', it is only 73 lm/W. It becomes obvious that '100lx' in the graph can have many many different values and, by definition, many different output currents for '100lx' unless lux is defined somehow.

A hint does occur elsewhere in the Hamamatsu document for a special lux-meter diode with mention of 'illuminant A' being approximated with a 100W light bulb.

I'm hoping that this thread will not be obfuscated by references to response vs. wavelength graphs this time

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Cheers,
Ted

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