Flash output power question (for school project)

02/28/2011 08:53:18 PM · #1

Hey all,

I'm woking on a satellite design project and one of our primary mission goals is to include a light beacon that flashes on a given interval and would be visible from earth with the naked eye. the luminosity (in watts) necessary for this type of visibility (rel. magnitude -3 aka same as the ISS) is 16 KW. at 1/1000 of a second flash time it means the flash tube must emit 16 Joules of energy.

does anyone know the joule output of say a Nikon SB600 per 1/1 flash pop? it says ~80 W-sec but the watt second (joule) rating of flashines is actually how much energy is stored in the capacitor. how efficient is the light generation process in these strobes?

any EE's here that can help? thanks so much!!

-Max

02/28/2011 11:19:35 PM · #2

The answer is "relatively efficient." If most, even half, of the energy were dissipated internal to the flash, the device would overheat badly within just a couple discharges. If you assumed an efficiency of about 80%, you probably wouldn't be too far wrong.
Another approach would be to use the "guide number" for the flash to calculate the emitted light energy, and use this to estimate the usable output. Warning: this calculation involves making a few assumptions, some of which can very materially affect the outcome. Personally, I'd stick with placing a lower limit on the output power as above.
A third, potentially fruitful path would be to build a crude calorimeter. Trap the output of the flash in a medium such as a liquid, and measure the temperature rise when the flash is discharged. Again, sounds easier than it is.

02/28/2011 11:21:58 PM · #3

A related question: did your luminosity calculation include the effect of a reflector directing the flash towards the Earth? If so, what did you use for an included angle? Remember that this will drastically affect the amount of power required.

Panasonic LUMIX DMC-LX100

03/01/2011 12:31:26 AM · #4

Thanks for the quick response Kirbic, yes we included a solid angle of 2Pi (hemispherical) since the thing is probably gunna be spinning (it has no ACS system) and we wanted to maximize the probability of seeing it. any way it only affects it linearly and were being conservative by using an intended apparent magnitude of -3 (ISS emmitance order of magnitude). we didnt account for the angle of bounce relative to earth (basically multiplying by cos of the vertical angle) but even if we see it every 3rd time thatd be fine.

the calorimeter idea is great, until now our only option for experimental measurement seemed to be an integrating cube (meh.)

some site spoke of a 50% radiant efficiency of xenon bulbs, does this sound right?

ETA:

also, is there a better light source for this kinda thing that you can think of? it needs to be really light, pretty efficient and have low power reqs (we'll be using a photo sensor to only pop it at night and charging wither a lithium cell or a NiCd by day by using maximum of like 5W of solar panels.

any ideas about dissipating the heat? we could make the reflector part of the chassis (intended to be milled out of a solid aluminum block) and hav it touch the bulb (obviously avoiding shorts)

any thoughts on UV degradation of the bulb?

i understand that this may be far too much to ask but i knew there were some EEs and MEs floating around here who might be able to throw a little knowledge my way

Thanks,
Max

Message edited by author 2011-03-01 00:36:11.

spiritualspatula

Nikon AF-S NIKKOR 24-70mm f/2.8G ED

03/01/2011 12:49:09 AM · #5

Are you sure a SB600 throws out 80w/s? The talk I've seen hasn't even been that high for an 800, which is typically pegged around 65 from what I recall. Or is that due to the difference that you note between power put into the capacitor vs output?

Panasonic LUMIX DMC-LX100

03/01/2011 01:15:36 AM · #6

Youre right. i looked more into power ratings and it looked like 30-50 for the 600 and at 50% efficiency thatd be 15-25 W-s or Joules. thats still enough for us. were looking for 15 J

spiritualspatula

Nikon AF-S NIKKOR 24-70mm f/2.8G ED

03/01/2011 02:03:14 AM · #7

Originally posted by michaelmonn:

Youre right. i looked more into power ratings and it looked like 30-50 for the 600 and at 50% efficiency thatd be 15-25 W-s or Joules. thats still enough for us. were looking for 15 J

Okay good, I just didn't want you beginning with what seemed to be inaccurate numbers. As far as heat dissipation, there are a ton of heatsink technologies out there. Look into the various designs for passive heat sinking on processors, for starters.

smardaz

03/01/2011 03:10:52 AM · #8

i don't know what you're talking about, may i still stand here?

03/01/2011 08:06:50 AM · #9

Originally posted by michaelmonn:

some site spoke of a 50% radiant efficiency of xenon bulbs, does this sound right?

It might be close. I'd have expected a little higher. Are you sure the site was talking about flash bulbs, not xenon filled halogen bulbs?

Originally posted by michaelmonn:

also, is there a better light source for this kinda thing that you can think of? it needs to be really light, pretty efficient and have low power reqs (we'll be using a photo sensor to only pop it at night and charging wither a lithium cell or a NiCd by day by using maximum of like 5W of solar panels.

If you were directing the beam, perhaps a pulsed laser. But the power you ne3ed is beyond any reasonable laser power. I think the xenon discharge tube is probably a good bet

Originally posted by michaelmonn:

any ideas about dissipating the heat? we could make the reflector part of the chassis (intended to be milled out of a solid aluminum block) and hav it touch the bulb (obviously avoiding shorts)

In darkness, heat dissipation will be very rapid (radiation) and you should not have to worry about it. Even on the night side, however, the satellite may be in sunlight. Heat may well be an issue then. If you are using a photodetector to supress flashes when the satellite is in sunlight, that should take care of the issues.

Originally posted by michaelmonn:

any thoughts on UV degradation of the bulb?

I don't think this will be a problem. It's basically a quartz tube with electrodes. The only thing UV may do is partially ionize the gas in the tube, but given that it's an inert gas, that's probably a stretch.

03/01/2011 11:42:10 AM · #10

Thanks Again Kirbic!

- we are using a photodetector to suppress pops in the daytime
- it will be in LEO (hopefully ~400 km) so no worries of constant thermal cooking
- thats nice to know that they may be more than 50% efficient :-P i guess we're being conservative
- i thought about a laser too but since there is no ACS onboard the probability that itd be pointing at us is slim

if we ever get this thing built/launched (at least a year just for logistics) ill be sure to alert any interested astrophotographers around here :)

Thanks,
Max