Measured light reception from 10W LED according to distance from centre
2013-09-19 by Robin Whittle
With the 10W UV LED I described in recent messages, I measured the
amount of light falling on a flat surface, like the PCB, according to
how far from the direct perpendicular point of illumination this is.
The LED consists of 9 chips encased in some kind of relatively soft
epoxy or silicone material. There is no side-ways attempt to reflect
light. The light comes out of the surfaces of the very thin chips
rather than their edges. I guess the conductive light emitting surface
is grown on sapphire, since it needs to conduct heat well and it cannot
be electrically conductive, since all the 9 chips are mounted directly
on the metal heatsink. So this is very different from 3mm and 5mm LEDs
where the chip tends to radiate sideways and have its light reflected by
a cupped part of the mounting electrode, and then further focussed by
the lens-like shape of the epoxy encapsulation.
As far as I know, and ignoring internal reflection from the surface of
the encapsulant, the whole LED assembly would radiate in all directions
evenly, with the energy varying by direction according to the Lambertian
pattern depicted as a circle in the diagram at:
http://zeiss-campus.magnet.fsu.edu/print/lightsources/leds-print.html
Looking at that diagram, I can see that at 30 degrees, the power is
about 88% of the power at 0 degrees. However, further factors affect
how much energy per unit surface area the PCB gets from the light at
such an angle.
1 - The PCB is further away than at 0 degrees.
2 - The light is falling at an oblique angle, so a given solid angle
of light radiation is spread over a larger surface area of Riston
or whatever photoresist is used on the PCB.
3 - The surface reflections will likely increase with angle from
whatever they are at 0 degrees (the light passing the surfaces
at 90 degrees to each surface):
Top surface of the glass (or whatever) cover.
Bottom surface of the glass cover.
Top surface of the phototool (I am using laser printed Mylar).
Bottom surface of the phototool (I am using laser printed Mylar).
Top surface of the thin Mylar protective film of the photoresist
which keeps oxygen away from the Riston. (Oxygen inhibits it from
polymerizing with light exposure.)
(This is probably very minimal, due to the likely close refractive
index match between the Riston and the Mylar protective film:
the bottom surface of the Mylar protective film.)
4 - The light travels through a greater distance of glass and phototool
than when the angle is 0 degrees from perpendicular.
I did some tests with a distance from LED to PCB of 34cm. The glass is
5mm conventional float glass - soda lime I assume. The glass is part of
an old 35mm film contact printing frame, in which the hinged glass cover
clips down over the film and photo paper, which sit on some soft
polyurethane foam. So this is probably higher quality (less iron and so
less green) than ordinary window glass, but I doubt if this matters much
for these measurements.
Below the glass is an unprinted sheet of the special "single matte" (it
is not even matte - it is almost gloss on one side and is gloss on the
other) Mylar laser film I use for phototools:
http://screenprinting.asc365.com/index.asp?ID=PD007003
Below that is my light sensor, a bare silicon solar cell, with only a
small section about 8mm in diameter exposed to light via a hole in some
black cardboard. This solar cell is unusual in that it has been
chemically etched so the surface resembles random pyramids. This is to
increase the light absorption. As far as I know this is a good way to
estimate the amount of light per unit surface area which would fall on
the Riston coating of a PCB in that location.
I measured the current in microamps and moved the open spot away from
directly underneath the LED, which was facing straight down. I moved
1cm per measurement. Here are the results with the left column the
fraction of the vertical distance I moved to the side and the right
column the percentage of light which fell on the exposed area compared
to the amount at 0cm = 0 degrees.
Distance Amount of light
0.000 100.0%
0.029 99.9%
0.058 99.3%
0.088 98.8%
0.117 97.8%
0.147 96.2%
0.176 94.3%
0.206 92.3%
0.235 90.3%
0.265 87.6%
0.294 84.9%
0.323 82.1%
0.353 79.1%
0.382 75.9%
0.412 72.7%
0.441 64.9%
0.470 66.5%
0.500 63.6%
0.529 60.4%
So with this 34cm distance, which will probably give me an exposure time
of 40 to 60 seconds, if I want a total range of exposure time being 10%,
I should keep the corners of the board within about 0.235 x 34 cm = 8cm
of the centre of illumination. That means I can do an 11.3cm square
board.
This is a pretty tight tolerance, since I think Riston has quite a wide
exposure latitude. If I allowed 20% less light at the corners, then
this would enable the corners to be ~0.343 x 34 = 11.7cm from the
centre. This is a 16.5cm square board.
I will do some careful exposure tests with a phototool with radiating
lines and spaces. The lines and spaces are the same width but their
width diminishes towards zero the centre of the radiation. When the
exposure is ideal, the resulting developed pattern in the Riston shows
the lines extending towards the centre without disappearing (under
exposure) or joining together (over exposure) better than for any other
exposure time.
- Robin http://www.firstpr.com.au/pcb-diy/