How to go from candelas to brightness?

[TheUnnamedNewbie]

So, I'm working on a project (game prop for airsoft/paintball) that involves a lot of lights and displays to make it seem complicated and like it is doing a lot.
In order to do so I want to use a number of 7-segment displays. Since this device will likely be used outdoor/in the sun during the day, I need it to be quite bright so it is nice and visible.

So that brings me to my question: having no experience with lighting and such, how do I know what kind of brightness I need, and how much candelas that translates to? Is 10 mcd a lot? what about 100 mcd?

[Feliciano]

Is 10 mcd a lot? what about 100 mcd?

The unit says it all: mcd = mili-candela (1/1000 of the brightnes of one candle).
After that, if you want to convert that to Lumens or Lux, you need to know how spot or flood is the cone that such lightsource project. (think of a candle inside a reflector).
You do the rest of the math.

[CatalinaWOW]

Unfortunately there is even more to the story.  You then need to compare it to sunlight, which is on the order of 100,000 lux.  Which will make the problem seem hopeless.  But you are not illuminating a spot on the ground with your display and trying to get that spot to compete with sunlight.  You are trying to create contrast with the sunlight reflected from the surround of your display.  So, except for pathological cases such as looking at a west facing display at sunset the sun is not normal to the surface and illumination efficiency drops accordingly.  In the pathological case the sun is going through a much longer atmospheric path and drops intensity a lot.  Add a little shade over the display and you can knock an order of magnitude off the solar intensity, maybe more.  You need to figure out the amount of light reflect that goes in the direction of the viewer.  And then compare that to the amount from the display that goes that direction. 

All a lot of computation, with many assumptions to be made. 

You may find it easier and far more informative to set up a simple outdoor test with a few LEDs with known specs and then just do relative calculations.  Important parameters to know about the LEDs are rated brightness, beam angle in both horizonatal and vertical directions, and how much of the visual field they fill.  Comparisons of the results will be much easier if the LED or LED array fills a resolution element of the eye (roughly 0.3 mR).

[PointyOintment]

These tables and accompanying explanatory text should de-confuse you on light units and "kinds of brightness" (because the word "brightness" does get used to refer to all sorts of different properties). (Yes, I know that's not what you meant by "what kind of brightness I need".) I particularly like this passage:

Many different units of measure are used for photometric measurements. People sometimes ask why there need to be so many different units, or ask for conversions between units that can't be converted (lumens and candelas, for example). We are familiar with the idea that the adjective "heavy" can refer to weight or density, which are fundamentally different things. Similarly, the adjective "bright" can refer to a light source which delivers a high luminous flux (measured in lumens), or to a light source which concentrates the luminous flux it has into a very narrow beam (candelas), or to a light source that is seen against a dark background.

Regarding how to actually achieve sunlight visibility, here's a SparkFun forum thread where it's discussed. Some suggestions from there are: experimentation, shade, contrast filter, blinking, point source, dark surround, LED sample kit, water clear LEDs.

Every clock radio uses a contrast filter. However, I rarely see them on outdoor LED signs and traffic lights, which instead usually use a dark surround and a large array of water clear 5 mm "ultrabright" LEDs.

[Feliciano]

You may find it easier and far more informative to set up a simple outdoor test with a few LEDs with known specs and then just do relative calculations.  Important parameters to know about the LEDs are rated brightness, beam angle in both horizonatal and vertical directions, and how much of the visual field they fill.

That's what would I do if I have some LEDs, drivers, and batteries. Other option would it be to inspect the construction details (physics and electronics) of a countdown pedestrian traffic light, and then think how small can you go from there.

[ajb]

What PointyOintment said.  Photometry is complicated. 

One specific thing to watch out for, try to avoid using a colored filter over the LEDs.  It's common to put, say, a red filter over a red LED display because in normal indirect lighting conditions the filter improves the contrast of the LEDs with the surroundings.  But in high intensity direct lighting, there's so much light coming in that the filter simply tints the light that bounces back out of the display and makes it look like the display is illuminated.  Water clear lenses with highly absorptive black surrounds do not have this sort of problem.  You see this problem on incandescent traffic lights that rely on colored filters when the sun is bright and low in the sky--the LED versions are much better in this respect.