Monday, March 4, 2024

Create Visible Light

In Gurps powers, the create energy advantage can create light... but it creates "1000 KJ" of energy. KJ is not how I usually measure my light, and its not how many people measure their light. So how much light does Create Visible Light [10] actually create? And how do you even measure that?

The radius lit in this table is lit with the officially recommended light level for indoor settings like bedrooms. I recommend using this as "no vision penalty"... though see the "nickpick" section for more details.

Radius Time Radius Time
1 yard 2 days 7 yards 1 hour
2 yards 12 hours 10 yards 30 minutes
3 yards 5 hours 15 yards 15 minutes
5 yards 2 hours 20 yards 7.5 minutes

x2 x1/4

Time listed is for each 1000 KJ of energy. If you have Create Visible Light 3 [30] and thus have 9000 KJ of light, a 5 yard radius light can last for 2 hours * 9 = 18 hours.

If you're not interested in being sure why this works, or about environmental complications that might make your game more interesting, you can stop reading here and just use the table. If not...


So where do these numbers come from? The biggest issue was figuring out a conversion from Joules to Lumen-Seconds. "Lumen" is a measure of light, and its sort of unique. The best way I can describe a lumen is to say a 100-Watt equivalent bulb delivers 1600 of them. If you got one of the legit ones and not a cheap one. 

And this lightbulb will do all of the rest of our work for us too. It delivers those 1600 lumens using under 16 watts of power, though not much under. So 1 Watt of power gives us 100 lumens-- and 1 Watt for 1 Second is one joule. So with an LED lightbulb, we can turn 1 joule into 100 lumens for a second, or into 1 lumen for 100 seconds.

So now we have lumens from joules, and we want to light a radius. Turns out there are official standards on how much light a given area needs. Using Gurps favorite measurement system, the measurement is the candle foot. You multiply your area in square feet by the recommended candle foot, and that's how many lumens you need. The recommended candle foot for living areas, bedrooms, and hallways is 20, so we can take the area of our radius, multiply by twenty candle foot, and that's the lumens we need. 

Then we divide the lumens by 100 to get the watts of power, and divide our 1 million joules by our required watts to get the number of seconds our light lasts for. 

So for a 1 yard radius we have about 3ft*3ft*pi = 28 square feet, and those 28 square feet require 28 sq ft * 20 candle foot = 560 lumens, and that requires 5.6 watts of energy, or 5.6 joules per second. our supply of 1 million joules will run out in 1,000,000/5.6 = 178,000 seconds. which is conveniently close to the 172,800 seconds in two days.

Nitpicks and Details

So this is not a perfect system, and I wouldn't be doing a good job if I didn't know where the weak spots in what I did are. 

First, we based things on commercial LED bulbs. LEDs are not perfectly efficient, and in fact, I used a convenient middle of the road number of 100 lumens per watt. Its easy to get 120 lumens per watt commercially, and those aren't perfectly efficient either: people claim LEDs are 70% efficient, which might mean 180 lumens is correct. Wikipedia claims the maximum optimized (green) light source can produce 683 lumens per watt... without a reference, and I'm a comparative newbie to optics. So I went with 100 for ease of use.

Second, these numbers are based on point sources in indoor lighting, because that's what people light up the most often. Indoor spaces have the benefit of bounding light off of all their surfaces to create nice soft light that lights most things up from all directions. When you stick a 1600 lumen bulb in a room, all 1600 lumens bounce around that room, especially if you painted the walls white. This uses all the light and ensures somewhat uniform lighting. 

Outside, or in a space larger than your light, you loose some of your light to the area outside. Worse, it doesn't bounce to light things up from behind, and you get stark modern lighting, which mostly just plays with near-point sources, we use mirrors, lenses, shapes, and multiple light sources to try and deal with this. Of course, nothing in powers restricts us to using only point-sources. light could be created in a downward beam over the area, or could appear ambivalently evenly from within the lit area. That could look pretty strange... which is another way of saying it would look mystical or cool. Not entirely inappropriate for a "Power". Reasonable limitation on the power might be restricting the light to appear as a beam, point source, or ambient. 

You may wish to impose lighting penalties just from the quality of the light, rather than the quantity. a sword fight in a searchlight beam at night may have a lot of light shining on it, but the shadows will be very harsh.

Third, 20 candle-foot is a little on the low side for lighting. Its equivalent to 200 Lux, which enhanced senses says is a -3 to vision rolls-- and then goes on to agree its typical for a living room. I don't usually run living rooms as -3 unless the lights have been intentionally dimmed. Lighting recommendations for normal areas generally range from 20-100, with high detail work areas occasionally getting a recommended value of 500 candle foot. Lighting for a stage also gets that recommendation.

While I don't think 200 LUX is worthy of a -3 penalty, perhaps I should increase the base lighting level to 50 candle-foot (500 LUX). That would drop the time by quite a bit. I'm not, because I'm hoping the efficiency error more or less cancels out. I might even in my games use an efficiency of 250 lumens per watt and an intensity of 50 candle-foot.

It's worth pointing out that 100,000 LUX (10,000 candlefoot) is the correct number for direct sun shining strait down with no clouds, and if you want to replicate that (it happens) you can get a 1 yard radius for 6 minutes, or a 10 yard radius for 3.5 seconds.


I made this to support a new campaign: we tried to play it about a year ago but the players wanted the magic system worked better... and here I am working the magic system. Create Light is a cool power (not just Create Visible Light, but all the things you can do with it), but I needed a better idea of what "Create Visible Light" could really do. I'm pleased with the result.

I'm happy to have this, and I've learned a lot. Light and Lighting is a complex topic, and I've only learned a little bit about it. If you have some way to improve this, let me know. I'm not running this game quite yet...

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