Light pen phototransistor recommendations ?

[shakalnokturn]

I need to replace a 3mm transparent case phototransistor in a light pen for the Thomson TO / MO series computers, what should I be looking for or avoiding?

[Gyro]

Depending on the screen resolution (pixel frequency) you might find a photodiode more appropriate, are you sure the original was a phototransistor?

[shakalnokturn]

I'll edit this when I've checked the resolution but it's low.
The schematic shows a phototransistor, no reference in the parts list though.

Edit: "Highest" resolution is 320x200 

[Kleinstein]

The electric internals tend to be quite similar. The main difference is likely with the optics. Chances are that a more focussed type is best suited. One should not get a type with IR fitler, but that should be obvious.  If in doubt one may have to test a few, as the normal plastic case phototransstors are cheap.

[shakalnokturn]

In the 3mm / T1 models looking at lowest possible centre wavelength or broadest spectrum and best focus, unfortunately most devices seem to peak in the I.R. even unfiltered.
I've selected a few readily available references, whatever the original was could be EOL now, the design is from mid '80s:

Vishay BPW85: 50°, Peaks at 860nm, 50% sensitivity at 620nm.
Vishay TEPT4400: 60°, Peaks at 560nm, poor focus with 72% sensitivity on axis and 100% when 13° off-axis...
Osram SFH-309: About 30°, peaks at 860nm, 50% sensitivity at 560nm.
LiteOn LTR-209: Well focused at less than 20°, no data for wavelength...

I'm leaning towards the SFH-309 or LTR-209 in the end it also depends on how cheap I can find them postage included.
Any suggestions for better models?

[Kleinstein]

The SFH 309 looks good and they even specify the speed. Some others seem to be slower (e.g. 15 µs).
There seems to be a balnce with speed and sensitivity, though not that much difference. If one has the choice probably test both a low and high sensitivity bin.

Having a peak wavelength of some 850 nm is normal for silicon sensor (phototransistor or photodiode). The phototransistors with a 550 or similar peak wavelegth would have an exter IR blocking filter to get closer to the human eye response. This is not really needed. The screen may also emit some IR that can be used.

I faintly remember using phototransistors as a light pen with the C64. AFAIR this were just the normal types, nothing special and it usually worked OK, provided the screen was bright enough and the cable not too long. If there is no amplifier in the pen, the cable capacitance could be an important point.