Instituto de Óptica “Daza de Valdés”

Evaluation of different general V(λ) mismatch indices of photometers for LED-based light sources in general lighting applications


  • The industry has new lasers affordable and stable femtosecond meters with a high repetition rate Photometers are used to measure the light intensity emitted by a light source

Madrid / 17 de January de 2023

When a new light bulb is designed and manufactured, it must be determined what type of light it emits and its quality. Today, almost only LED-based light bulbs and light sources are evaluated.
In evaluating these light sources, photometers are used which must be calibrated using a known incandescent bulb, and it is very likely that the CIE standard illuminant A (the incandescent bulb) will soon be replaced by the CIE L41 reference spectrum ( White LED with around 4100 K).
graph showing two curves, one shaped like camel humps and the other rising

Spectral distribution of the two illuminants, SA(λ) and SL41(λ) (also called SL(λ), respectively.

This replacement has raised the question of whether an update to the mathematics behind the calibration index of photometers, called the general mismatch index V(λ), is necessary.

In this work, an international team has compared other alternatives to the general mismatch index V(λ) to assess whether any of them might be more suitable when using the CIE L41 reference spectrum (white LED) as calibration spectral distribution, due to the disuse of conventional bulbs compared to the use of led bulbs.

Why it is necessary to calibrate photometers

Humans do not see all colors in the same way, our sensitivity varies from one color to another and that is why photometers must be adjusted to the so-called photopic vision to “see” like us.

Two graphs showing how blue looks better at night and red during the day

Sensitivity to colors during the day (photopic vision) and sensitivity at night (scotopic vision) / Eng. Alex Ramírez, Consultant, 2008 (www.bun-ca-org)

The difference between the sensitivity of the photometer and ours is what is measured by the general mismatch index V(λ) and this index is defined in relation to a standard incandescent light source.

After the study, the authors have concluded that the current definition works very well even with the future change of the standard light source and it is not necessary to change it.
Other indices evaluated for a possible new index definition perform only slightly better in some cases, which does not justify replacing the current index, which is widely accepted in the international community.

What is photopic and scotopic vision

Photopic vision is the sensitivity we have to colors in broad daylight. With night light the sensitivity changes and is called scotopic vision.

There are two types of vision because in the eye we have two types of receptors, cones and rods.
Daytime vision, photopic, is based on the response of the cones. The cones need much more light to activate than the rods, so they only activate when the lighting levels are high enough.

In addition, there are three types of cones, each with different levels of sensitivity towards colors. This allows the human visual system to distinguish between different colors.

The cones are able to distinguish details better than the rods, which populate the retina less densely.

This has been a joint research work in which the following institutions have participated: the company TechnoTeam Bildverarbeitung GmbH from Germany, the CSIC Institute of Optics, the Aalto University Metrological Research Institute of Finland, the Technical University of Denmark, the Institute of Light Technology of Germany , the Metrology Department of the Portuguese Quality Institute and the Physikalisch-Technische Bundesanstalt of the German National Metrology Institute

Related news