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  Experimenter's Corner

  Previous Issues

Testing LED Light Intensity
By: Dave Johnson

 

The human eye is not a good instrument for measuring how bright a light is.  Our eyes tend to be logarithmic.  When making a comparison between two lights, unless one light is considerably brighter than the other, you will not know which is really brighter.  You need a linear device to make such a measurement.

You donít have to spend a lot of money to make good intensity measurements.  A fairly accurate device is the bare silicon photodiode.  As the cart below illustrates, a silicon photodiode has a broad light response, covering invisible ultraviolet, through the entire visible spectrum, into the invisible near infrared band.  These parts act as small solar cells.  The DC current generated by them is directly proportional to the

light intensity.  By positioning one of these things directly in front of a light source, you can accurately measure how bright one part is with respect to another.  This method does not take into account the emission pattern of the part, or its color.  It only tells you what the relative intensity is.  It works best when you are comparing one device with another of the same color, package size and emission pattern.  By placing the photodiode very close to the LED under test, the emission pattern has less impact on the intensity reading.  Most of the light emitted, strikes the photodiode.
The most popular LED package is the 5mm diameter T 1 ĺ part.  The position of the LED chip within the epoxy package controls the light pattern that emerges.  If the chip is mounted back from the lens, the pattern is narrow.  Conversely, if the chip is moved closer to the lens, the pattern is broader.  The standard LED specification for the emission angle is the angle from the center of the part to the half power point.  This is a half angle.  

Some LED data sheets double this angle for a full angle.  So, a device with a 10 degree half angle may be listed as having an emission pattern of 20 degrees.  Keep in mind that when viewed from some distance away, a LED with a narrow emission pattern will appear brighter than one with a broader pattern, even if the broader pattern part emits more light power.

LED with a Narrow Pattern   LED with a Broad Pattern
When testing T 1 ĺ type LEDs, I reach for my photodiode test tool.  As illustrated, this assembly is just a photodiode slipped into a black plastic tube about 0.65 inches long.  The LED under test slides into the other end of the tube.  The black plastic prevents most ambient light from reaching the photodiode.  I then connect the photodiode to my multimeter, set for the DC current mode and a 20ma range.  The current reading I get on the multimeter is then the light intensity. 

The photodiode I use most often use is the PDB-C142 from Advanced Photonix Inc.  This device costs about $3.00 and is available from www.digikey.com.  Other possible devices are shown below.  In a pinch, you can even use a small solar cell.  When using a bare solar cell try to make your measurements under low ambient light levels.  Otherwise you will get a false reading.  When picking a photodiode, be sure to only use those with a clear window.  Donít use photodiodes with a purple tint.  These contain a special daylight blocking filter which filters out most visible light.  Also, be sure to only use a photodiode.  Phototransistors are often listed as light detectors but they are very logarithmic parts and will not give you a good linear response.

Advanced Photonix PDB-C142   BPW34 BPW46  PDVC158  BPV10

When testing LEDs, the best way to drive them is with a constant current.  Twenty milliamps seems to be the industry standard for most LEDs.  You can approximate a constant current by inserting a resistor in series with the LED and a DC power supply.  You can use the formula below for picking the resistor value.  Red LEDs have a forward voltage between 1.5v and 2.0v.  Orange and yellow require about 2 volts.  Green parts need about 2.5v to 2.8v.  Blue and white LEDs need something between 3.2v and 3.6v.  Nearly all ultraviolet LEDs need about 3.6v while nearly all infrared LEDs need about 1.5v.

Small Solar Cell
Over the years I have been very disappointed by how poor some white LEDs used in many solar path lights and flash lights are.  They may look bright but by using this simple test tool, the intensity measurements tell a different story.  Some white LEDs made by Cree are two and sometimes three times brighter at the same current level.  If you are not satisfied with a particular path lightís output, you can make a big difference just by replacing the LED with a better part from Cree.  Also, if you are satisfied with the path lightís intensity but not the time it operates, you can replace the lightís LED with a Cree part and extend the operating time by 2X or 3X by increasing the internal resistor which controls the LED current and thus lowering the drive current.  Flashlights too can benefit by swapping out the LEDs for better parts.  I have also used this simple setup to monitor how some parts age over time.  Some cheap white LEDs offered on eBay fade to a half intensity level in just a few weeks of use.

Please send comments to me


First Quarter,  2012    

 


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