How this calculator works
Radiance measures power per effective source area and solid angle. For an ideal Gaussian beam its characteristic value is power divided by wavelength squared. Circular beams with equal M² on both axes have an additional (M²)² factor in the denominator. This is an optical beam metric, not a prediction of perceived colour brightness or laser exposure limits.
Formula
L = P/[(M²)² λ²] in W m^-2 sr^-1; photon flux = Pλ/(hc). For an ideal Gaussian beam M² = 1.
Worked example
A 1 W beam at 1000 nm with M² = 1 has radiance 10^12 W/m2/sr. A 1 W beam at 500 nm has four times that radiance under the same convention.
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Frequently asked questions
Is higher power always higher radiance?
Only when wavelength and beam quality are unchanged. A more divergent beam can have lower radiance despite more power.
Does this compare visual brightness?
No. Human spectral sensitivity and viewing geometry are not included.
Does a passive lens increase radiance?
An ideal passive optical system preserves radiance while exchanging beam size and angular spread.