How to measure the LED's wavelength without Spectrometer device - Page 1
Forum section: electronics / beginners

author: elefurtronik
author_rank: Regular Contributor
timestamp: January 14, 2023, 12:15:34 am
content: Hello,I am curious about whether is there any way to measure a LED's wavelength by filtering the infrared wavelength range?I just want to make a small circuit to measure it. I don't want big spectrometer devices to measure that.

author: james_s
author_rank: Super Contributor
timestamp: January 14, 2023, 02:28:20 am
content: I don't understand the question. What do you mean "filtering the infrared wavelength range"? If you want to measure the wavelength you generally need a diffraction grating and a slit to shine the light through.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 14, 2023, 02:47:32 am
content: A lot depends on how accurately you want to know. Narrowband filters (on the order of 15-20 nanometers) are available. Except for a few common wavelengths they would be special order and expensive, but small and light. A go-nogo test will identify whether the receiver operates in that band.You can also get lowpass and highpass optical filters. A simple variant of the half interval sorting method can narrow down the spectral range of your source.Finally you can make you own crude spectrometer with a small grating or ir transmissive prism. It will take a bit of calibration but needing be the room filler that some conventional spectrophotometers we're.In any case you will likely want to use a lock in amplifier. Basically you pulse your light source and synchronously filter the receiver. A little googling of those terms should get you lots of circuits. They are not daunting in complexity.

author: elefurtronik
author_rank: Regular Contributor
timestamp: January 14, 2023, 12:17:51 pm
content: Quote from: james_s on January 14, 2023, 02:28:20 amI mean there are two LEDs next to each other one is infrared and another is normal red LED light. I want to measure only red light's wavelength. However, this red LED can change to different colors, as well. Therefore; my main purpose is to filter this infrared wavelength and blackout it. After that, I will measure other LED's wavelengths. +/-15-20nm tolerance is acceptable for me.Quote from: CatalinaWOW on January 14, 2023, 02:47:32 amWhat electronic components can measure LED's wavelength? Could you share some examples for the low-pass and high-pass optical filters? I think those optics should be big. I need a tiny solution. :/

author: CaptDon
author_rank: Super Contributor
timestamp: January 14, 2023, 01:46:58 pm
content: Or say "I'm reverse engineering a medical SPO2 sensor".

author: CaptDon
author_rank: Super Contributor
timestamp: January 14, 2023, 02:02:22 pm
content: A small slit detector and a prism would work. Move the slit or rotate the prism to see the detectors response. Pulsing the LED and looking at your detectors response on an oscilloscope would be good. If you can't pulse the LED then put a chopper wheel in the path. Your color changing LED may be a combination of only two LED's, a ruby red and an orange or yellow. The color can change from red normally through to what the eye perceives as orange or yellow/orange but in reality it is done with only two fixed color LED's plus an I.R. LED. It will be hard to calibrate a home built prism device unless you have some LED's whose spectra is known, then you can interpolate.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 14, 2023, 02:36:30 pm
content: I was referring to optical filters (Google it). They can be very small. Millimeter dimensions. Narrow band ones pass only appropriate frequencies of light. Put one matching the wavelength of your IR LED over your detector and you won't see the visible LED. Same thing if you use a low pass optical filter that cuts off the visible and shorter wavelengths. If needed these filters can be moved in and out of the optical path with a solenoid or filter wheel. The low pass optical filters are widely used and should be cheap, possibly even free if salvaged from an old projector or other device where they were used to allow thermal radiation to escape while recovering the visible output of incandescent bulbs.If you want to electronically measure the wavelengths it is possible in principle to add a super stable laser, heterodyne it with your light source and measure the difference frequency. Although simple in concept this is very hard to actually implement, and is much larger than a refractometer.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2023, 02:44:58 pm
content: There are photodiodes / phototransistors available with integrated filter. Many detectors made specificly for IR have a filter suppress light below some 750 nm. This may just be enough to keep red out, at least many of the newer LEDs.There are Photodiodes with an anit_IR fitler to approximate the eyes sensitivity - these don't react much to the IR.So some degree a red LED can work as a photodiode and would also not react to IR light. It depends on the type how well they work.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 14, 2023, 04:09:33 pm
content: If you have the part number you read the data sheet. Else you befriend someone who works in optics.

author: elefurtronik
author_rank: Regular Contributor
timestamp: January 14, 2023, 08:26:01 pm
content: Hello,Thanks for all the suggestions. First of all, I am not reverse engineering anything.Second, I need to get a measurement electronically and trigger some codes. From this perspective, I will check photodiodes and phototransistors. I hope I can find what I need.

author: nali
author_rank: Frequent Contributor
timestamp: January 14, 2023, 08:37:28 pm
content: If you mean you want to do it on the cheap, try this perhaps?