Why in professional spectroscopes diffraction gratings are used instead of prisms?

Question

Why in professional spectroscopes diffraction gratings are used instead of prisms?

I guess transparency is not an issue, as we need optics anyway.

Answer 1

Just compare the resolution of the two:

• Prism depending on n, there is no good material n>1.7 (besides diamond)
• depending on base length if you use a equilateral triangle
• have to use more than one to overcome this
• prism absorb light, you have got scattering (stray light) too

Now a grating:

• optimize it for your wavelength
• choose lines per milimeter
• resolution depending on the number of lines that are illuminated
• compact device
• just transmission gratings have got absorption, you can do your measurement in reflection with a blazed grating
• design your blazed grating to get the most light in e.g. 2nd order

Quantitatively

Prism: $\frac{\lambda}{\Delta \lambda} = t \frac{dn}{d\lambda}$

Grating: $\frac{\lambda}{\Delta \lambda} = \frac{zD}{g}=zN$

where t is your base length, z... order of spectrum, g...grating constant, D...entrance beam diameter, N...number of illuminated lines

So just use a grating, nowadays they can be fabricated in excellent quality. On my university learning the pros of a diffraction grating is part of the 1st year laboratory exercises.

Answer 2

I found a comparison of the two possibilities and it seems that prisms are better.

This manual for a spectroscope used for jewelry stones, gives higher marks to the diffraction grating towards the red part of the spectrum, where the crystal gets lower efficiency. It seems the efficiency for the grating is uniform for the whole spectrum. It is also possible that economically it might be cheaper to use a grating than three crystals, as this particular brand proposes to solve the efficiency for crystals.