I need to make something that reads brightness value of an LED from voltage output of photodiode.
I will be using the following parts:
OSRAM OSTAR LE UW E3B datasheet
- Forward voltage Vf (typ.): 20.8 V at forward current If = 700 mA
- light emitting surface: 2.1 mm x 3.2 mm
- Luminance: 22 x 10^6 cd/m^2
- optical efficiency: 65 lm/W at 350 mA
Hamamatsu Si-PIN diode S6036 datasheet
- Reverse voltage Vr: 12 V
- lens: phi 7 mm
- sensitivity: 0.56 A/W
- short circuit current: 30 uA
- dark current: 0.1 nA
I will see the voltage output as drawn in this figure
And voltage input range of my ADC equipment is within 2 volts peak to peak and +/- 3 volts.
I need to know which resistor I need to use, or whether I need put amplifier or attenuator, in order to adjust output voltage level from photodiode towards ADC input.
To figure out it, here's my calculation:
sensing area of photodiode: 3.14 * (0.007 / 2) ^2 = 3.848451e-5 m^2 # 0.007: lens phi candela value into sensing area: 22e6 * area = 846.65922 cd candela-Watt conversion: 1 cd = 18.3988 mW over a complete sphere watt value: 15.57751 W current output from photodiode: watt * sensitivity = 8.7234 A assume that + 2 volts is the maximum acceptable, 2 V = 8.7234 A * R thus, I should use resistor with R = 0.2292684 ohm
But, I think 0.23 ohm is absurdly small.
Since I'm not familiar with electrical engineering, circuitry and optical, I have totally no idea whether my calculation is correct or not, and whether I use the terms in the equations with right interpretation.
Another problem is, I think current output from photodiode towards ADC does matter. If it is too high or low, ADC will be fried or breakdown due to too much current drain. I'm not sure that it is correct or not. If it is wrong, please fix the statement. Back to the question, anyway, is it correct that the current matters for ADC?