CD diffraction grating

[ZiHao]

Hello,

I am planning to build a shoe box CD grating with DSLR+18mm kit lens to analyse the spectrum of different light sources and hopefully someone will share their thoughts on the design.

I am not sure about the number of lines per mm of the grating, maybe next week I will buy a laser to calculate the value but let's assume it's 625 lines/mm. I calculated the length of entire spectrum which is about 2.4cm and not sure whether this has a direct relationship with the sensor size or not, which is 2.35cm long.

Any help will be appreciated. 

Edited March 20, 2021 by ZiHao

[saac]

I can't comment on your design but you may find this site helpful; it offers some useful resources including some a free web based analyser where you can upload and share your spectra. 

https://publiclab.org/wiki/economist

 

Jim 

[vlaiv]

If you have lens in front of your DSLR than you should not think in terms of linear units - you should think in terms of angular units.

violet light: sin(theta) = 0.2375 => theta = arcsin(0.2375) = ~13.739 degrees

red light: sin(theta) = 0.4375 => theta = arcsin(0.4375) = ~25.9445 degrees

spectrum is spread over 12.2 degrees

Now if you want spread that spectrum over sensor with 0 order you need to fit 0°-26° on sensor horizontally - so your DSLR (I'm assuming APS-C sized sensor here) should use 46mm focal length

If you want to fit only spectrum on sensor, then you need to capture 12.2° - and you can use 100mm lens.

With 18mm lens - you'll get 63.3° over horizontal and this means that zero+first order will span about 1/3 of sensor, while spectrum itself will span about 1/5 of sensor.

In any case, if you have 18-55mm kit lens - you will be able to find suitable focal length that will capture both zero order and spectrum nicely, or only spectrum - but up to half of the sensor size.