peter p
04-21-2002, 11:57 AM
Does power wattage work with light in the same way as it does with sound eg: logarithmically (spelling?)?
To double intensity of 50watt bulb we require 500watt bulb?
IP: Logged
MrKnobs
Senior Member
Posts: 952
Registered: Dec 2001
posted April 20, 2002 08:50 PM
--------------------------------------------------------------------------------
Tempted to give you the short answer, but I think it would just raise more questions. Read this first, if you like, then we can talk better.
http://www.intl-light.com/handbook/ch01.html
Terry D.
IP: Logged
agedhorse
Member
Posts: 585
Registered: Dec 2001
posted April 20, 2002 08:54 PM
--------------------------------------------------------------------------------
Yes and no, it's similar, though our perception sensitivity curve is different. In lighting, it's in part due to the fact that we are dimming by changing the voltage as a percentage, and power is Vsquared/R, the response is not linear. double the voltage and the power quadruples due to the squared factor.
Same in sound, double the output voltage of the power amp and the power quadruples. The heat dissipation by the output stage also quadruples, that's why you are now seeing 2 ohm versions of some popular amps. They lower the rail voltages slightly whivh dramatically alters the deat the amp generates... at the expense of less 8 and 4 ohm output.
IP: Logged
MrKnobs
Senior Member
Posts: 952
Registered: Dec 2001
posted April 20, 2002 09:34 PM
--------------------------------------------------------------------------------
You asked two questions. The answer to your first is unequivocally yes. The answer to your second, is not so simple.
The reason there is no "correct" simple answer to the question (although there is a rule of thumb) is that brightness has no units, it's a perception. People often use the terms "lumens" and "watts" to mean brightness, but both those terms are actually power.
Which is why I referred you to the link. You will see the first part of the puzzle, which is Planck's equation relating power to wavelength. While you read this, keep in mind that there is no non-coherent lighting source I know of that produces the same spectra at different power inputs. When you crank it up, the color changes.
Now let's mix in the response of the human eye. The eye is definitely non-linear in terms of brightness vs. power, and decidely non-linear in perception of brightness vs. wavelength, with maximum response somewhere in the yellow/green region if I remember my physiology classes correctly from college. Also you've got that pesky little iris stopping down in a semi-successful attempt to reduce all bright scenes to the same level. The more you crank up the power, the more everyone's pupils become pinpoints.
It's also a relative thing: the darker the stage, the brighter the lights will be perceived to be as everyone's eyes adjust. Like in a movie theater.
So, there is an answer relating power in to (light) power out, but that depends on the light source. I don't believe that there is a simple answer to power in vs brightness out, though there is certainly a rule of thumb that I have heard many times.
I think it would be interesting if you also posted this query on the lighting forum to see what range of answers you get.
Terry D.
[This message has been edited by MrKnobs (edited April 20, 2002).]
To double intensity of 50watt bulb we require 500watt bulb?
IP: Logged
MrKnobs
Senior Member
Posts: 952
Registered: Dec 2001
posted April 20, 2002 08:50 PM
--------------------------------------------------------------------------------
Tempted to give you the short answer, but I think it would just raise more questions. Read this first, if you like, then we can talk better.
http://www.intl-light.com/handbook/ch01.html
Terry D.
IP: Logged
agedhorse
Member
Posts: 585
Registered: Dec 2001
posted April 20, 2002 08:54 PM
--------------------------------------------------------------------------------
Yes and no, it's similar, though our perception sensitivity curve is different. In lighting, it's in part due to the fact that we are dimming by changing the voltage as a percentage, and power is Vsquared/R, the response is not linear. double the voltage and the power quadruples due to the squared factor.
Same in sound, double the output voltage of the power amp and the power quadruples. The heat dissipation by the output stage also quadruples, that's why you are now seeing 2 ohm versions of some popular amps. They lower the rail voltages slightly whivh dramatically alters the deat the amp generates... at the expense of less 8 and 4 ohm output.
IP: Logged
MrKnobs
Senior Member
Posts: 952
Registered: Dec 2001
posted April 20, 2002 09:34 PM
--------------------------------------------------------------------------------
You asked two questions. The answer to your first is unequivocally yes. The answer to your second, is not so simple.
The reason there is no "correct" simple answer to the question (although there is a rule of thumb) is that brightness has no units, it's a perception. People often use the terms "lumens" and "watts" to mean brightness, but both those terms are actually power.
Which is why I referred you to the link. You will see the first part of the puzzle, which is Planck's equation relating power to wavelength. While you read this, keep in mind that there is no non-coherent lighting source I know of that produces the same spectra at different power inputs. When you crank it up, the color changes.
Now let's mix in the response of the human eye. The eye is definitely non-linear in terms of brightness vs. power, and decidely non-linear in perception of brightness vs. wavelength, with maximum response somewhere in the yellow/green region if I remember my physiology classes correctly from college. Also you've got that pesky little iris stopping down in a semi-successful attempt to reduce all bright scenes to the same level. The more you crank up the power, the more everyone's pupils become pinpoints.
It's also a relative thing: the darker the stage, the brighter the lights will be perceived to be as everyone's eyes adjust. Like in a movie theater.
So, there is an answer relating power in to (light) power out, but that depends on the light source. I don't believe that there is a simple answer to power in vs brightness out, though there is certainly a rule of thumb that I have heard many times.
I think it would be interesting if you also posted this query on the lighting forum to see what range of answers you get.
Terry D.
[This message has been edited by MrKnobs (edited April 20, 2002).]