SFH 3310 & SFH 5711 Light Sensors

May 18, 2009

in Sensors, Tinkering

After I successfully shed some light its time to see some light. For this you normally use photo diodes, photo transistors or photo resistors. But there are two nice little detectors which I came across, special enough to post them here:

The Osram SFH 3710 and SFH 5711. Both are photo diodes respectively transistors adapted to the human eye sensitivity (V?). They are very insensitive in the infrared spectrum (typical for humans, untypical for photo detectors) and have a detection curve close to the human eye. But more details after the click.

SFH 3710

The SFH 3710 is more or less a very normal photo diode. Nothing special about it. It outputs something between 4 µA and 12 mA depending on the amount of light shining on the sensor. Unfortunately it has current output. So you need an sensing resistor to ground to get some voltage (you remember the law of Mr. Ohm).

But it hast two very special advantages:

  1. It got a very nice 0805-like tiny SMT package. So it consumes very little board space.
  2. But much more important it has a similar detection curve as the human eye.

The last point is very important (so I used it for may ‘How was your day darling‘ project). Normal photo detectors are very sensitive in the infrared spectrum, the humane eye is not. What is the effect? Artificial light (especially from normal light bulbs) have an high amount of infrared light and a low amount of visible light (One of the reasons for the inefficiency of light bulbs). For normal photo sensor the light of a light bulb seems to be as bright as daylight (figuratively spoken).

So if you want to detect light in the same way as it is perceived by an human the SFH 3710 is your friend. BTW: The SFH 3310 is the same detector in a more or less ‘normal’ T1 (3mm) LED package.

SFH 5711

The SFH 5711 is quite a different beast. It is an photo transistor. Nothing special about this. It also got an V? curve. Fine. But it is logarithmic. To understand the advantages of an logarithmic light sensor you have to understand how the human eye works:

The human eye interprets lights in logarithmic levels. If you see an light level X than 2*X will not appear twice as bright, X*X appears twice as bright. This enables the human eye to see things in a dark cave and bright sun light. To understand how light something appears to a human you have to apply the logarithmic scale.

This is exactly the work done by the SFH 5711. It outputs an current from 5 to 50 µA – depending on the light level. But it puts out about 10 µA per decade (of brightness). The details are a bit complex – but the result is quite easy: If the readout is quite as big the light will be interpreted as twice as bright by the human eye. So you get an more or less constant resolution over the whole spectrum of brightness – as perceived by the human eye. Very useful to adapt the brightness of a display according to the ambient light levels.

The disadvantage: The SFH 5711 contains an amplifier to get an readout – so you have to suplly VCC (from 2.3V to 5.5V) and GND. The sensor is bigger and needs a special footprint. In the end still very practical.

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