Photometry
Understanding how light is measured, represented and interpreted in an optical system.
From light to useful information
What are we actually measuring?
Photometry is concerned with measuring visible light in a way that reflects the response of the human visual system.
For optical engineers, that provides a practical way to quantify the result of combining a light source with an optic. It allows us to move beyond simply describing an optic as narrow, medium or wide and look at how luminous intensity is distributed through space.
That distinction matters because an optical system is rarely judged on one number alone. The application may need light concentrated in a particular direction, spread across an area, controlled at certain angles or distributed with a particular degree of uniformity.
Describing visible light
Three measurements you'll encounter regularly
Different photometric quantities describe different aspects of a lighting system. They are related, but they are not interchangeable.
Luminous flux — lumen (lm)
Luminous flux describes the total quantity of visible light emitted by a source, weighted according to the sensitivity of the human eye.
Luminous intensity — candela (cd)
Luminous intensity describes how strongly light is emitted in a particular direction. This is especially useful when examining how an optic redistributes the output from an LED.
Illuminance — lux (lx)
Illuminance describes how much luminous flux reaches a surface. One lux is equal to one lumen per square metre.
Seeing where the light goes
Reading a luminous intensity distribution
A polar plot provides a visual representation of luminous intensity at different angles around the optical axis.
The centre represents the origin of the measurement and the angular scale shows direction. The plotted curve indicates the measured intensity at each angle. A narrow distribution produces a tighter central shape, while a wider distribution extends further across the angular range.
The important point is that the curve describes the distribution, not simply a nominal beam width. Shoulders, secondary peaks, asymmetry or changes in the rate at which intensity falls away from the centre can all influence how the beam behaves in an application.
A useful reference, not the whole story
Beam angle and FWHM
One common way to describe beam width is Full Width at Half Maximum, or FWHM. The peak luminous intensity is identified and the angles at which the intensity has fallen to 50% of that peak are located. The angular distance between those two points gives the FWHM beam width.
Why it is useful
Using a defined measurement point gives engineers a repeatable way to compare the width of different distributions.
Why it is not enough
Two distributions can share a similar FWHM while differing significantly outside the 50% intensity points. The complete distribution therefore remains important.
Performance belongs to the combination
Why the LED matters to the optic
An optic does not have one fixed photometric performance independent of the light source beneath it.
The size and geometry of the LED's light emitting surface, its position relative to the optic and the way light is emitted can all influence the final distribution.
This is why photometric data is most useful when the LED and optic combination being considered is clearly defined.
Same optic, different LED
Changing the LED can alter the resulting intensity distribution even though the optic itself remains unchanged.
Same LED, different optic
Changing the optic changes the way light from the same source is controlled and redistributed.
Confidence in the data
Why the test setup matters
Photometric comparison depends on consistent measurement conditions. Positioning, alignment, the LED-to-optic relationship and the test geometry can all influence the result.
Repeatable setup and measurement methods allow engineers to compare one combination with another with greater confidence and help ensure that differences in the data relate to the optical system rather than unintended changes in the test arrangement.
Positioning
The LED and optic need to be located consistently relative to one another.
Alignment
The optical axis and measurement geometry need to be understood and controlled.
Repeatability
Consistent methods allow meaningful comparison between different LEDs, optics and measurement runs.
Turning measurement into engineering decisions
Start with what the application needs
Photometric data becomes useful when it is interpreted against the requirements of the finished system.
Rather than asking which optic has the highest intensity or the narrowest beam, the more useful question is whether the complete distribution meets the needs of the application.
Consider the target area, working distance and useful angular range.
Spill light, glare and unwanted intensity at particular angles can be just as important as the central beam.
Some applications benefit from a strong central peak; others require a smoother and more uniform distribution.
Published optical performance should be considered alongside the specific source used in the finished system.
Positioning, available space and integration can influence the optical relationship.
Define the required optical outcome before focusing on a particular component.
Talk to the engineering team
Have an optical requirement to work through?
Photometric data is most useful when it is considered alongside the LED, mechanical arrangement and requirements of the finished application. Our engineering team can help evaluate those relationships and work through the available optical options.
Talk to Carclo Optics →