LED uniform lighting

Question

I need to provide uniform intensity or uniform spread of light using this LED in a particular area(Small region). I have read about diffusers such as engineered diffuser, ground glass diffuser, Light shaping diffuser etc. Which is good for my project? Which diffuser can provide highest efficiency?Small region means the dimensions are length x width x height : 5cm x 5cm x 3cm(rectangular box). Sample is placed at the bottom centre of the box and led is placed 2.7cm away from my sample at the height of 1.5cm

Answer 1

I don't believe that uniform light intensity within that rectangular box is possible with only one LED. If I would have to do it, I would try a box made from special frosted plexiglas lighted by an array of LEDs outside with a distance of about 1 cm from each LED to the next LEDs and a distance of about 1 to 2 cm to the box. Of course each side of the box should be lighted with an array of LEDs.

Answer 2

The best practical way to do this is to use a small LCD display module which incorporates an LED backlight. The LED backlight in most of these displays include both the LEDs and the diffuser. These are very common, and very cheap these days. You can probably find one in a discarded piece of electronic gear.

There are two possibilities. First way: you can remove the image-producing LCD glass from the metal frame and simply use the LED backlight and diffuser which are usually a mechanically separate assembly located behind the LCD glass.

Second way: you may find that if you power up the displays backlight LEDs, and leave the LCD section un-activated, that the LCD display will go into "transmissive" mode which means it remains perfectly transparent. The whole screen will appear to be uniformly white. It is in fact emitting a very uniform white light. This may not be readily apparent unless you darken the room.

On most of these small LCD displays the LCD and LED connections are separate and thus easy to identify. The LCD section is usually connected via a ribbon cable type connector with a dozen or so signal connections. (In your application you would leave these completely unconnected.) The LEDs are usually connected with a single pair of small gauge discrete wires (often red & black).

The LEDs are usually electrically connected in a series-string, or a combination of parallel connected series-strings. Since the LEDs in 99.9% of these units are white LEDs, the forward voltage drop of each is around 3.2 volts. Therefore, each series string in the LED backlight will have a forward voltage drop which is a multiple of this voltage. Typically, around 12 to 20 volts will ignite the backlight once you get the polarity correct. Don't worry if you get the polarity backwards, these are pretty resilient to reverse polarity failure. Use a current-limited bench supply to power the string, set the current limit low (50 mA typical ) and slowly raise the voltage until you see light. Don't go above 20 volts. If you don't see any light reverse the polarity of the connections. Hint: if a black wire is utilized, it is likely the cathode connection to the string, which you would connect to the negative side of your power supply. Also, keep the room lighting dim so you can see the LEDs as they begin to ignite dimly.

Once you identify the correct polarity, and the approximate required forward voltage, do not try to run the LED to full brightness with a direct connection to the power supply. Insert a current limiting resistor of appropriate value and wattage in either feed-wire connection, then experiment with higher voltage levels to determine the maximum achievable brightness of the LEDs.

A final caveat. Earlier generation LCD displays utilized high-voltage fluorescent tubes for backlights. Such LCD displays are not useful for your application, and none of the above information applies to this type of display.

Answer 3

One way to get reasonably uniform light onto the interior of one box face is to put lights at the opposite face. These lights are spread out over the surface so that the light is fairly uniform by the time it reaches the opposite face.

However, if that's all you do, the edges will be lit less well than the middle. The trick to get around this is to put mirrors on the remaining four sides. That takes away the edges of the lit area. If you were to look from the box face that is supposed to receive light and look towards the opposite side, you'd see a infinite array of lights. This provides even illumination from a sufficient distance.