7-Segment display brightness?

[MrPuhur]

I'm replacing old VFD displays with LEDs, since the former were incredibly fragile and broke one after the other.

So there I was, looking for LED 7-segment displays at Farnell when I noticed that the Luminous Intensity of the displays varies widely, form 500μcd to almost 3cd, or 6000 times more.
I really have no sense for the numbers or any physical reference other than a 650μcd (from the datasheet) display I've got in the parts bin. It's quite bright, even though not enough for this one.

What would be the optimal range of candelas I should choose from? It's a piece of industrial equipment mostly used in a well-lit workshop and I have to buy 27 of them, so I can't really 'go safe' as price somewhat correlates with the brightness.

[David Hess]

Going to need specific examples.  Within one series, the big difference is color.

[MrPuhur]

Here's the one I'd have chosen if I hadn't noticed the brightness ranges. It has a lumnious intensity of 2.9mcd:
http://uk.farnell.com/multicomp/703-0163/display-green-0-56-1-digit-an/dp/2112184


These are a bit more expensive, the first has a luminous intensity of 26mcd:
http://uk.farnell.com/kingbright/sa39-11sykwa/display-seven-segment-9-91mm-yellow/dp/2373461
This is smaller compared to the others. This might have something to do with it, considering I read that candelas take in account the 'solid angle' the light is projected to.

This has 4.7mcd:
http://uk.farnell.com/kingbright/sc56-11ywa/led-display-0-56-yellow-cc/dp/1142444

I likely have to use green around 555nm, since as far as I know it is the colour the eye is most sensitive to in a bright environment.

The datasheets of the Kingbright ones actually don't have the same numbers as shown in the product page, as the test was done with 10mA of forward current, not the 20mA they're likely to be used at. This, and the relative luminous intensity graphs only got me even more confused.

[Paul Moir]

I just replaced a couple of those kingsbright ones on a temperature chart.  They aren't lying: they're about a bright as you would possibly ever want. 

[Gyro]

Some additional information on how you're going to drive the displays would be useful. I'm sure you know that LED isn't a drop-in circuit replacement. VFDs usually have dedicated driver ICs. How you're doing the conversion could affect the safely available current.

On the brightness, it's always better to err on the bright side - you can always drop the brightness with higher value series resistors. There's not much you can do to brighten up excessively dim ones without running out of drive current.

[MrPuhur]

The circuit itself is going to be quite simple, as the digit and segment signals are essentially the same. The original VFD was driven using high voltage buffer chips, which had 5V inputs. Both the segment and digit signals were driven using the same circuit, as VFDs need the them both HIGH for the respective segment to light up. I'm taking these 5V signals before the HV buffers, adding my own 5V buffer to the digit side and an inverting buffer to the segment side. That is if the LEDs happen to be common anode. The power is going to be provided by the 5V logic supply through buffers, so I don't I'll be limited in terms of available current.

I just replaced a couple of those kingsbright ones on a temperature chart.  They aren't lying: they're about a bright as you would possibly ever want. 

Which ones exactly?