LED pulsing and strobing

LEDs can be used to make continuous light, keeping constant the current flowing through the device.

On the other hand, LEDs can be easily driven in a pulsed (on/off) regime and can be switched on and off in sequence, turning them on only when necessary.

Usage of LEDs in pulsed mode has many advantages:

  • Extension of their lifespan.
  • Reduction of the power dissipation.
  • Reduction of the generating heating.

If the LED driving current (or voltage) is set to the nominal value declared by the LED manufacturer for a certain amount of time and then is reset to zero, we talk about pulsed mode: the LED is simply switched on and off.

LEDs can also be driven at higher intensities (i.e. overdriven) than the nominal values, thus producing more light but only for a limited amount of time: in this case we say that the LED is operated in strobed mode.

Strobing is needed whenever the application requires an increased amount of light to freeze the motion of fast moving objects, in order to eliminate the influence of ambient light, to preserve the LED lifetime and to synchronize the ON time of the light (ton) with the acquisition time of the camera and item to be inspected.

To properly strobe an LED light, a few parameters must be considered (see figure):

  • Max pulse width or ON time (ton): the maximum amount of time for which the LED light can be switched on at the maximum forward current.
  • Duty cycle D is defined as (usually expressed in %):

`D=t_(on)/(t_(on)+t_(off))`

Where toff is the amount of time for which the LED light is off and T = ton+toff is the cycle period. The duty cycle gives the fraction in % of the cycle time during which the LEDs can be switched on. The period T can be also given as the cycle frequency f = 1/T, expressed in Hertz (Hz).

Duty cycles parameters.
Duty cycles parameters.
Triggering and strobing.
Triggering and strobing.

How to determine the maximum ton for different strobing frequencies?

The key point in driving the LED in strobed mode is not to overcome the maximum power rating of the LED. The power dissipated by the device is expressed by the following equation:

`P_(diss)=V_(averag e)*I_(averag e)`

This equation is easily computable when dealing with continuous light illumination. On the other hand, when the LED is strobed, the turn-on and turn-off times must be taken into account. Typically Opto Engineering illuminators are delivered with all the LED strobing characteristics (see figure).

Strobe Specs
Product strobing specs

For example, considering the LTPB illuminator shown in the figure above, the maximum driving current is 1.8 A and the maximum ton time is equal to 1ms when strobing at 15 Hz. In this case the maximum average current of the LED is equal to:

`I_(averag e,max)=I_(max)*t_(on)/(t_(on)+t_(off))`

Remembering that the strobing frequency is:

`f=1/T=1/(t_(on)+t_(off))`

the current can be computed as:

`I_(averag e,max)=I_(max)*t_(on)*f`

Or, in another way, using the definition of duty cycle D:

`I_(averag e,max) = I_(max)*D`

Substituting the maximum driving current, the on-time and the strobing frequency, we obtain:

`I_(averag e,max)=27 mA`

This current MUST NOT be exceeded when using this illuminator.

If, for example, we want to strobe at 30 Hz (double with respect to the previous frequency), it is mandatory to reduce the on-time to 0.5 ms (half of the previous on-time), so that the product of the maximum driving current (1.8 A) with the new two data (30 Hz and 0.5 ms) doesn’t exceed the value of 27 mA.

In summary, the advantages and the disadvantages of strobing LED sources are the following:

ADVANTAGES

DISADVANTAGES

A large amount of light can be obtained for a short period of time (mandatory for fast application)

A light controller is necessary in order to strobe the LED source properly

Increase the lifespan of the LED

The synchronism between illumination and camera acquisition must be guaranteed

Can reduce the power dissipation

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