# Apollo Mission Control Room 2, how do these decimal number displays work?

In a recent question there was a link to this arsTECHNICA page.

One of the pictures of digital clock displays:

How do these decimal number displays work? I guess there are assembleys of a mask with the digit, a lens and a lamp for each of the 10 digits. So each of the very beautiful number fonts are projected to the backlight screen. It is amazing how sharp and precise these images are.

• Ever seen a Rolodex? (hint, this technique (if it's what I think it is) only works for monotonically changing values like clocks. Think 1970's "digital" alarm clocks before 7-segment LED displays)
– uhoh
Commented Jun 29, 2019 at 13:45
• @uhoh check the other images, they also used the same type for non-counting displays: cdn.arstechnica.net/wp-content/uploads/2019/06/… Commented Jun 29, 2019 at 14:00
• @asdfex oh, well there goes that theory then.
– uhoh
Commented Jun 29, 2019 at 15:54
• wait a second... is this stop watch in the top left actually at 39:61? Damn leap seconds! Commented Jun 29, 2019 at 16:32
• @asdfex good catch about 39:61
– Uwe
Commented Jun 29, 2019 at 16:36

I'm not sure about this precise type of display (*), but there are ones very similar and that exactly match your description: Ten individual light bulbs, ten lenses and a mask with cut-outs for each digit.

They were called "in-line displays" or "projection displays" and became popular around 1960, before they vanished in favor of LED-based 7-segment displays.

Here you can find a video of one of these displays "in action": https://i.sstatic.net/I5Nvv.jpg

Here's also an dismounted device with all components visible: http://www.industrialalchemy.org/articleview.php?item=511 (As there is no license given, I'm not going to repost these images here)

(*) They look slightly off to me, on the image it seems they are just reflecting light and are not lit by themselves. Could be that this is a different type, or the original displays have been replaced for the exhibition. Here's an original image of the control room, taken from Wikimedia Commons:

In the bottom left corner you can spot several numeric displays like these, but they are definitely lit and as to be expected from these kind of display, the digits are not as well aligned as the ones in the rebuilt.

• wow, what a beautiful design, cool! Yes the ones in the recent images look to flat or matte to be projections onto ground glass or some diffusor, it looks like an updated technology of some kind or even just a painted simulation, but the original ones must be something like what you are linking to.
– uhoh
Commented Jun 29, 2019 at 15:58

I'm pretty sure that the numbers in RETRO's chronographs are stickers (or nice LEDs), but not connected to actual clocks etc (the leap-second gives it away).

The original photo from Wikimedia Commons shows much more clearly what look to me to be Nixie tubes. The slight misalignment of the baseline between numerals when seen off-axis is a strong indicator, as each digit would be at a different depth in the tube.

Given that NASA procurement was on government contract to lowest bidder (mostly), I doubt NASA would have used something as expensive as in-line displays. In addition, the mentioned in-line displays would be less reliable (predictable life) and require more maintenance than standard Nixie tubes that had been used for decades already.

My vote is Nixie tubes in the original, and stickers in the restoration display.

• See this image found by Hobbes. It is a different look to Nixie tubes.
– Uwe
Commented Jul 2, 2019 at 13:26
• Those are definitely not Nixie tubes. Seeing that, it wouldn't surprise me if there were some of both on consoles -- the in-line displays for larger more important measurements (like mission elapsed time?) and Nixie for less critical? I also notice that another of the displays in the ASTP photo above (indicating "05") does look like an in-line display not a neon Nixie, and the display above to its right looks like something in between: wrong color for neon (although not out of range) and not as crisp as the "05" display. Commented Jul 2, 2019 at 13:36