Hello...
Robin can you test what time takes if the led is just 1 cm above
(instead 34) board?
I know the area will be small but I'm just interested how big efect the
distance have on time?
If source is point then relation is square from distance but the led
isn't point source...
Thanks.
On 09/19/2013 09:06 AM, Robin Whittle wrote:
> 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/
>
>
>