It sees only infrared
The black package stops visible light. The PT334-6B answers from about 760 to 1100 nm at half strength or better, peaking at 940 nm, which is the TK63's colour and that of most TV remotes. So a white LED lamp barely registers, while sunlight and filament bulbs, rich in infrared, read high. The black plastic is a visible-light filter, not a daylight filter.
Black, on purpose
The dome is black for the same reason a TV's remote window is dark: the plastic is dyed to stop visible light and let infrared through. The maker's curve starts at about 710 nm, past the deepest red you can see, reaches half strength at 760 nm, peaks at 940 nm and falls back to half at 1100 nm.
Pick TK63 emitter. Its infrared is a narrow band at 940 nm, right on the sensor's peak, and nowhere near what the eye sees. Almost all of it counts. The two blocks were picked for each other.
Pick White LED lamp. A white LED is a blue chip under a yellow phosphor, and it gives off almost nothing past 750 nm. The sensor answers to almost none of it. A room lit by LED lamps is, to this block, nearly dark.
Sunlight is the problem
Pick Sunlight. About half of the sun's light is visible and the plastic stops it, but a large share of the rest falls between 760 and 1100 nm, and the sun is so bright that this share alone can outshine a small emitter. A filament bulb is worse for its size: most of what it gives off is infrared. Both read high, and both can take SIGNAL to the ceiling.
The shares in the figure are worked out from textbook spectra, not measured, and they say what fraction of each light the sensor answers to, not how bright any of them is.
So
Point it away from windows and bulbs, or shade it. For a beam, use the TK63, whose colour is the sensor's peak, and measure the room with the emitter off before you trust a reading with it on. A beam that breaks does exactly that.
When it does not work
Sunlight is full of infrared, and the black plastic lets that part through. Shade the sensor with a short black tube pointing at the emitter, keep it out of direct sun, or use the break-beam sketch, which subtracts the room's reading from the beam's.
A phone torch is a white LED, and a white LED gives off almost no light past 750 nm. To this sensor it is nearly dark. Test with a TV remote, a filament bulb, or the TK63.
Your eye cannot see 940 nm. Most phone cameras can: look at the TK63 through the camera app and a working emitter shows as a faint glow. Some phones filter it out on the main camera; try the front one.
A little. The datasheet's typical curve shows the current rising from about 88 % at 0 °C to about 126 % at 70 °C of its value near room temperature, so a sensor in the sun reads a few per cent higher as it warms. Compare readings taken close together in time, not days apart.
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