How does a light grid work?

Functionality of DUOmetric light curtains and light grids

An infrared light grid, also called a light curtain, consists of several transmitter and receiver elements. In principle, they are light barriers strung together.

The transmitter emits infrared light beams that can be detected by the receiver. Infrared LEDs with a wavelength of 880-940 nm are used for the transmitter. Photodiodes or phototransistors are typically used for receivers. These are able to detect the incoming infrared light and convert it into an electrical signal.

When an object interrupts the light beam, the receiver is triggered and a signal is sent to the controller.

Switching behavior

There are two different switching behaviors: Light switching and dark switching

Dark switching:

Switching output is active when beams are interrupted

Light switching:

Switching output is active when all beams are free

Application options

Light grids are used in a wide variety of industries and applications.

Detecting light grids are used for simple tasks such as counting large objects or checking the presence of objects and persons.

In more complex applications, such as volume measurement, measuring light grids with external controller are required.

Multiplex operation

Depending on the size of the object to be detected / measured, a suitable beam spacing needs to be selected. The beam spacing of a light grid is the center-to-center distance between two adjacent infrared beams. We offer beam spacings from 5 mm to 112 mm.

When using small beam spacings adjacent transmitter and receiver elements could see each other due to the light cone emitted by the transmitter elements.

To prevent the neighboring beams from influencing each other, only one beam at a time is active and is evaluated (sequential evaluation or multiplex operation).

Due to the high speed at which the beams are evaluated, there is practically a closed light curtain.

Depending on the controller and range, per beam cycle times starting from 6 µs. The range varies from 200 mm up to 25 m.


If two light grid systems are mounted next to each other, reflections can also lead to objects not being reliably detected.

In this case, it is necessary either to operate both light grid pairs with one controller, or to synchronize two controllers (one used for each pair) accordingly.

Beam variations

physical beams or diagonal beams?

In the light grid, both physical and diagonal beams can be evaluated. With activated diagonal beams, smaller objects can also be detected with a larger beam spacing. However, here the object can also be detected reliably in the entire monitoring area, because the resolution varies depending on the position of the object.

In the central area, the object can

also be detected with the diagonal beams

near the transmitter and receiver bar, the object

can’t be detected

Therefore, the evaluation of only parallel beams is more reliable with a smaller beam spacing. When using only parallel (physical) beams, the range between transmitter and receiver has no effect on reliability and accuracy.