On Thu, 15 Nov 2001, Harvey Perkins wrote:
> As a very inexperienced sometime-amateur spotlighter, can anyone provide me
> with an informative synopsis of the relationship between wattage and
> candlepower?
Watts are a measure of power. In this case they are measuring electrical
consumption by the spotlight. Spotlights used for birding seem to be
in the range 10-100 watts.
Candlepower are a measure of light intensity. I believe the candlepower
quoted for spotlights is for a point in the centre of the beam, i.e. it
indicates how bright the centre of the spot will be. Spotlights used
for birding seem to be rated in the range 50000-1000000 candlepower.
I've not sure how accurate these ratings are.
You can't convert watts to candlepower unless you know the efficiency
of the bulb, the efficency of the reflector and the shape of the beam.
For example, I have a 25 watt spotlight with a narrow beam and a 50 watt
spotlight with a wider beam. The centre of the 25 watt spotlight's beam
is brighter and it would hence have a higher candlepower rating.
The candlepower rating should indicate how well the spotlight will
illuminate an object in the centre of the beam at a given distance.
The candlepower divided by the distance in metres squared should give
you the illumination in lux.
At 10 metres a 1000000 candlepower spotlight will illuminate an object
in the centre of the beam with 1000000/(10*10) = 10000 lux - similar to
a cloudy day. At 100 metres it should be 100 lux - similar to indoor
house lighting. At 1 kilometre it should be 1 lux - a little brighter
than moonlight.
A prescription to avoid harm to fauna by limiting spotlights to 30 watts
is vacuous. The limitation should be in lux or perhaps lux hours.
For example, other things being equal a 30 watt spotlight will be
brighter at 10 metres than a 100 watt spotlight at 20 metres. In other
words, you should worry more about distance than watts.
My personal opinion, for what little its worth, is that spotlighting has
an impact on fauna but the intensity of illumination isn't significant at
typical ranges. I doubt the impact from spotlighting is significant,
except in locations when spotlighting occurs very frequently.
Several years ago, Peter Woodall posted Jack Pettigrew's opinion on this
topic: http://menura.cse.unsw.edu.au:1080/1997/04/msg00006.html
While I'm rabbiting on, The watts rating should give you a good idea of
power consumption. A popular spotlighting power source is 12V 7.2Ah
sealed lead acid batteries. These nominally produce 12*7.2 = 86 watt
hours. This means a 40 watt spotlight will run nominally for 86/40 =
2 hours.
In practice, you'll get less than that depending on discharge rate,
temperature, battery age and how well you've treated the battery.
As Simon Brock mentioned, you shouldn't discharge sealed lead-acid
batteries below about 11.5V. Generally if you notice the spotlight
begin to yellow/dim, its time to stop using the battery.
Lead-acid batteries have a longer life if kept charged. It also helps if
they are stored in a coolish place (< 25 degrees). They self-discharge
over period of months. If you are not using your spotlight I presume
its better to recharge the battery once or twice a year.
If you are transporting sealed lead-acid batteries be very careful to
ensure that the terminals can't be shorted - they contain sufficient
energy to produce intense heat and start a fire. Covering the terminals
with electrical tape is a good precaution. For obvious reasons, you
can't transport sealed lead-acid batteries as checked airline luggage.
They seem to be theoretically permissable as hand luggage but I suspect
you're still likely to be refused permission to carry the size of
battery used for spotlighting.
Andrew Taylor
Birding-Aus is on the Web at
www.shc.melb.catholic.edu.au/home/birding/index.html
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