Skip to main content

Vision-Doctor

Illumination

Illuminate features & make defects visible

Flashing / Strobing of LED illumination

LED lightning controllers are the ideal complement to LED lighting. They offer many advantages when operating the lighting and save costs and maintenance time in the long term:

Benefits of flashing LEDs for industrial machine vision

Current control of the LED allows precise control of illumination. Constant currents ensure constant brightness. This can be used both in flash mode and in controlled continuous operation with the aid of a flash control unit.

In both pulsed mode (<=100% output) and oberstrobe mode (several hundred percent output), the lighting is switched off most of the time. 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 heating LEDs

Temperature effects

Flashing significantly increases the life of the lighting. The effects of ageing 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 much higher than the temperature that can be felt outside the luminaire housing.

Precise over-flashing of the illumination with a multiple of the power increases the luminance efficacy. The consequence is either a shorter exposure time in order to better freeze any object motion or the use of a higher focal ratio for more depth of field using the same exposure time.

More light also automatically means a reduction of the interference from the outside. Problems with daylight, for instance, can be avoided in this way.

Junction Temperatur und Lebensdauer von LEDs

Principle of the current control of an LED

A technically very useful principle for overstrobe, pulse and continuous operation of LED lighting is current control. LEDs are basically current-controlled and there is a proportional relationship between current and brightness. This means that 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, so a 1% change in voltage can result in a 10% change in brightness. The forward voltage also changes during operation, when heated and permanently due to ageing. Unwanted short and long term brightness variations are the result. Unstable power supplies can also cause such brightness fluctuations.

Blitzstrom und Lichtintensität

Eine LED kann unter bestimmten Bedingungen ein Vielfaches an Licht emittieren, ohne Schaden zu nehmen. Je höher der Blitzstrom, desto mehr Licht wird emittiert.

How strong can an LED be overstrobed?

  • Rated power in continuous operation
  • LED characteristic
  • Pulse duration
  • Overall cycle time
  • Operating temperature

There are examples of complex diagrams for various LEDs that provide information about the flash intensity. The adjacent table shows these relationships in a highly simplified form.

As a rough guide, an LED with a total duty cylce of approx. 5 % and a single pulse duration of 0.5 ms can be flashed with five times the constant current and emit approx. 2.5 times the amount of light.

Overstrobe factors and permitted duty cycles

Important:

The uncontrolled overflashing of LED lighting can lead to damage to the illuminant. Please ask your machine vision expert for more detailed information for the correct operation, who will select suitable illumination and flash controller!
Please thoroughly acquaint yourself with the current safety regulations if strongly flashing light is used in the system. Perhaps shielding would be useful in this case.

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)