Making a phototransistor behave equally in sunlight

ACharnley · 2025-01-10T10:33:54-0800

Just after some quick thoughts.

I've a photo transistor which was specifically chosen to block infrared. This it appears to do, however during day[sun]light the read is double that of a head-torch shining on the sensor. As a quick test I held a lighter flame close and it blows out the sensor read.

I'm thinking I need to filter the red and possibly yellow component, so a blue filter. Does this sound correct?

Nigel Goodwin · 2025-01-10T10:47:00-0800

ACharnley said:
Just after some quick thoughts.

I've a photo transistor which was specifically chosen to block infrared. This it appears to do, however during day[sun]light the read is double that of a head-torch shining on the sensor. As a quick test I held a lighter flame close and it blows out the sensor read.

I'm thinking I need to filter the red and possibly yellow component, so a blue filter. Does this sound correct?

Sun light is massively brighter than a torch, so it sounds like it's behaving exactly as it's supposed to.

schmitt trigger · 2025-01-10T13:32:03-0800

Why would you want to filter out some colors? The sunlight is full spectrum, whereas any artificial light, specifically the fluorescent and LED ones, will have significant spectral gaps.
Incandescent lamps are also continuous spectrum, but leaning very heavily towards the red.

schmitt trigger · 2025-01-10T13:36:49-0800

Figure 4 you posted indicates that the detector’s maximum sensitivity occurs around 625 nm, which is a reddish-orange. Small wonder that a flame causes it to react.

ACharnley · 2025-01-10T14:41:55-0800

Yeah, and it's not reacting the same under what the eye thinks is the same brightness indoors vs outdoors. Whence I thought to filter the redy orange out.

Cant recall having this with LDR's but then I checked and they usually react more towards the blue wavelength.

ACharnley · 2025-01-10T14:45:27-0800

Sunlight is misleading, even overcast at dusk it's at 100% while in the garage under an led strip light it's at 50% ish. Both I would say are about equal brightness wise so it has to be spectrum.

schmitt trigger · 2025-01-10T14:54:39-0800

Yes. It is the spectrum content. As a fact, the CIE defines the 6500K daylight white as the standard illuminant.

Making a phototransistor behave equally in sunlight

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Making a phototransistor behave equally in sunlight

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