# Photodiode : Spectral Range Vs Wavelength

#### odm4286

Joined Sep 20, 2009
241
Hello everyone, I'm putting together a BOM for my next project. It involves detecting light emitted from a 650nm laser diode. I'm a little confused on the spectral range and wavelength attributes of photodiodes. Should I ensure the spectral range of my photodiode covers the wavelength of my laser diode? Or, do I need to match the wavelength of the photodiode with the wavelength of my laser diode. Thanks!

#### jpanhalt

Joined Jan 18, 2008
7,848
Hello everyone, I'm putting together a BOM for my next project. It involves detecting light emitted from a 650nm laser diode. I'm a little confused on the spectral range and wavelength attributes of photodiodes. Should I ensure the spectral range of my photodiode covers the wavelength of my laser diode? Or, do I need to match the wavelength of the photodiode with the wavelength of my laser diode. Thanks!
What is the difference? The range of wavelengths detected by the photodiode will not be nearly as narrow as the wavelength spectrum produced by the laser. All you need is a photodiode that will respond to the laser wavelength.

John

#### odm4286

Joined Sep 20, 2009
241
Hmm, on digkey there is a photodiode that has a wavelength of 580nm but a spectral range of 330nm - 720nm. Is it safe to assume spectral range is just another way of saying "detection range"?

#### jpanhalt

Joined Jan 18, 2008
7,848
The sensitivity of photodiodes varies with wavelength. It is not necessarily a simple geometric curve. In any event, the photodiode will give some response within that stated range. If one needs to accurately measure the photons, then there are ways to calibrate the photodiode for a particular wavelength. I do not think photon counting is in the TS's plan.

John

#### DickCappels

Joined Aug 21, 2008
5,873
Below is the sensitivity curve of the Rohm BH1680FVC-TR. Its peak sensitivity is 530 nm, but notice that ots sensitivity is down only 20% at 650 nm. Something with a response like this should work fine with your laser.

#### bertus

Joined Apr 5, 2008
20,040
Hello,

Hamamatsu has some nice PDF's about the subject of si photodiodes.

Bertus

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#### Papabravo

Joined Feb 24, 2006
12,383
I would say that since spectral response is given as a ratio you could compute the 3 dB down points as follows:

$-3 = 10 \times log(\text ratio)$
$10^{-0.3}\approx 0.5$

Says me any ratio above 0.5 should be good or 450 nm to 700 nm

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#### hp1729

Joined Nov 23, 2015
2,304
Hello everyone, I'm putting together a BOM for my next project. It involves detecting light emitted from a 650nm laser diode. I'm a little confused on the spectral range and wavelength attributes of photodiodes. Should I ensure the spectral range of my photodiode covers the wavelength of my laser diode? Or, do I need to match the wavelength of the photodiode with the wavelength of my laser diode. Thanks!
650 nm is red? Do you expect interference with normal lighting?
.

#### odm4286

Joined Sep 20, 2009
241
650 nm is red? Do you expect interference with normal lighting?
.
Haven't put much thought into it, this is mostly a learning exercise at the moment. I do not have any experience with laserdiodes or photodiodes. Should I anticipate some false positives due to ambient light?

#### Papabravo

Joined Feb 24, 2006
12,383
The photodiode response will be proportional to the power applied to the area. I think you will find that the laser diode has a much higher intensity than the ambient background. If you want you can place the photodiode at the end of a opaque black tube which will limit the range of incidence angles of the impinging light.

#### DickCappels

Joined Aug 21, 2008
5,873
Haven't put much thought into it, this is mostly a learning exercise at the moment. I do not have any experience with laserdiodes or photodiodes. Should I anticipate some false positives due to ambient light?
Maybe. There are many techniques to reduce interference from ambient light. Optical bandpass limiting might help some in this case, restricting the field of view of the receiver, pulse coding or pulse modulation so that the signal out of the detector can be detected after passing through a bandpass or pulse matching filter, but since you are planning to use a laser diode, your best bet might be just to reduce the receiver's sensitivity and take advantage of the fact (?) that the intensity of the laser beam is stronger than any ambient light falling on the sensor.

My preferred method when there is interference so to switch the LED (in my case,) on and off at several KHz, bandpass the output signal and then detect the signal, followed by a filter.