Benefit for industrial machine vision when strobing LEDs
The current control of the LED enables precise control of the illuminance. Constant currents guarantee constant brightness. This can be used both in flash mode and in regulated continuous operation with the aid of a strobe control unit.
The lighting is switched off most of the time both in cyclical operation (<=100% output) and in overstrobe mode (several hundred per cent output). Heating is avoided and the spontaneous light output of the LED remains constant at 100% at an operating temperature of 25 degrees oC.
Light reduction when LEDs heat up
Temperature effects
Pulsing significantly increases the service life of the lighting. Ageing effects and the associated loss of intensity are significantly reduced.
Caution: Technically, only the temperature at the LED semiconductor crystal (junction temperature), where most of the heat is generated, is of interest. This is considerably higher than the temperature that can be felt outside the luminaire housing.
Precise over-strobing of the lighting with a multiple of the power increases the luminous efficacy of the lighting. This leads either to shorter exposure times in order to better freeze the movement of the object, or to the use of a larger f-number in order to achieve a greater depth of field with the same exposure time.
More light automatically means less interference from outside. Problems with daylight etc. can thus be avoided.
Junction temperature and LED lifetime
Prinzip der Stromregelung einer LED
Current control is a technically very useful principle for flash, pulse and continuous operation of LED lighting. LEDs are basically current-controlled and there is a proportional relationship between current and brightness. Doubling the current will almost double the light output. If the current is kept constant, the light output remains constant.
An alternative to controlling the intensity of LEDs is to control the voltage. Small changes in voltage result in large changes in intensity, e.g. a 1% change in voltage can result in a 10% change in brightness. The forward voltage also changes during operation due to heating and permanently due to ageing. The result is unwanted short and long term brightness variations. Unstable power supplies can also cause such brightness fluctuations.
Current and Luminous Intensity
The maximum permissible lightning current depends on several factors:
There are exemplary complex diagrams for different LED which provide information on how strong the flash may be. The below table illustrates these correlations in a clearly simplified way:
As a rough guide, an LED with a total cycle time of approximately 5% and a single pulse duration of 0.5 ms can be flashed at five times the constant current and emit approximately 2.5 times the amount of light.
Overstrobe factors and permitted duty cycles
Need machine vision lighting ?
Vision-Doctor.com is a private, independent, non-commercial website project without customer support.
For the best advice, training and sales of machine vision lighting please click here.
(external link partner website in Europe)
