A light source has an Irradiance of about 1.6 W/m²/nm at wavelenght 385nm and an LED:

Peak Wavelength 385nm Viewing Angle 30 ° Luminous Intensity 80 mcd

If this light source shines on a circular area of 0.01m² how many LED's would be required to shine on the same area to provide an equivalent Irradiance at 385nm?

I have tried to illustrate the matter below. uv led


Optical geometry.

Emitter: 5mm D LED say 65mW ~ 100mW
Assuming +/- 30 deg Lambertian response below half power
Target: 0.01m² = 100 mm x 100 mm square or some equiv round area.

Calc: LED Emittance= input (0.1W) Pwr * ζefficiency(?) / Area/ wavelength[nm] = 0.1Wζ/385 nm/π{2.5 mm}²/ = 13.25 * ζ W/m²/nm

Irradiance ratio of spreading light from π*{2.5 mm}² into 0.01 m² = 0.01/1.96e-5= 509

Thus 1 LED = 13.25ζ/509 but you need 1.6 so you need 1.6*509/13.25ζ= 62/ζ LED's but ζ may be only 10% ??


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