Lava flow (Kilauea, 2012)

Broadband radiometric measurement of light-emitting diodes with a photodiode

idonus offers a wide range of LED-based UV exposure systems. These products are an efficient and durable alternative to mercury lamps which have traditionally been used for UV photolithography and curing.

With what similarity can the spectrum of a mercury-vapour lamp be reproduced with LEDs? Mercury-vapour gas-discharge lamps have a continuous spectrum with characteristic strong lines at 365.4 nm (i-line, UV-A ultraviolet), 404.7 nm (h-line, violet) and 435.8 nm (g-line, blue). On the other hand, LEDs have an easily recognisable Gaussian-type spectrum. LED semiconductors can be constructed to emit light at virtually any desired peak wavelength. By combining arrays of LED dies emitting at well-defined wavelengths, it is possible to construct a broadband high-power LED that mimics the useful spectrum of a mercury lamp. Moreover, the relative intensity of these peaks can be tuned independently.

To what extent can a high-power UV-LED replace a mercury lamp in a photolithography process? The question arises when considering the overall spectral irradiance and the output irradiance in the useful UV range. Understanding the origin of the spectral behaviour of LEDs, as well as the reasons why a simple photodiode can be used as a reliable radiometric instrument should be of interest to those considering replacing their old mercury-based system. It should also spark the interest of those already familiar with the use of UV-LEDs for their processes. These topics are fully covered in the White Paper that we have just released and which we are promoting in this newsletter.

Spectral power distribution of a mercury-arc lamp versus that of a tunable broadband UV-LED

Spectral power distribution of a typical mercury-vapour lamp compared to that of a tunable broadband UV-LED. 

Read below

Hereafter, we give a brief overview of the content of our White Paper.

Glowing electrical blackbody

Electrical glowing blackbody, Lummer and Kurlbaum (1898).

Blackbody radiation law and the birth of quantum mechanics

At the end of the 19th century, scientists at the Physikalisch-Technische Reichsanstalt (PTR) in Berlin laid the foundations of radiometry. Their electrical glowing blackbody (see photograph) is an iconic instrument of that time because it allowed blackbody radiation to be measured with unparalleled accuracy. On the basis of measurements completed by Rubens and Kurlbaum with that equipment, Planck established his famous blackbody radiation law which laid the foundations of quantum mechanics [Pais, 1982].

Subtle is the Lord, Abraham Pais

"Subtle is the Lord: The Science and the Life of Albert Einstein," Abraham Pais, 1982.

Photodiode and LED photonic devices

Quantum physics is the basis of semiconductor physics. It allows us to understand the spectral characteristics of photonic devices, particularly photodiodes and light-emitting diodes (LEDs).

At idonus, we just released a White Paper where we give a glimpse into the quantum physics behind photodiodes and LEDs. The subject is approached with an historical perspective, starting in 1900 with Planck's law and ending with the 2014 Nobel Prize in Physics.

The spectral power distribution of a light-emitting diode (LED) has a distinctive asymmetrical gaussian shape. This is the macroscopic expression of finely tuned properties buried inside the semiconductor.

To understand how a photodiode can be used as a reliable radiometric instrument for the characterisation of broadband LEDs, download our White Paper. The key idea is to use prior knowledge of the centroid wavelength of the emitted light and account for it to calculate the responsivity of the photodiode.

doi: 10.13140/RG.2.2.34765.05600/2

Read more

Click on the link, to get direct access to the PDF "preview version" of our White Paper. 

If you would like to read the full version, simply send us an e-mail at

Please contact us at:

Phone: +41 32 724 44 40

idonus sarl, Rouges-Terres 61, 2068 Hauterive, Switzerland