How to read a tube data sheet

A proper night vision device comes with a data sheet. One page, usually a table, carrying the serial number of the tube fitted and the values that were actually measured at acceptance.

Most buyers put that sheet aside unread, and that is understandable: it looks like a test report from a laboratory, because that is exactly what it is. It is also the only document in the entire purchase that says something about your device rather than about the model. Twenty minutes with it are better invested than any product description.

Here is what the lines mean.

First: model figure or measured value

This is the most important distinction of all, and it appears nowhere on the sheet.

Manufacturer figures for a tube type quote minimum values and sometimes typical values. "SNR min. 28" means: no tube of that type leaves the factory below 28. Your particular example may have 28, or it may have 34.

A data sheet tied to a serial number quotes measured values. That is something else entirely, and it is the reason you should ask for one.

If the sheet you are handed carries round minimum values throughout and no serial number, then you are not holding a test report, you are holding a brochure.

Signal to noise ratio (SNR)

What it is: how clearly the image stands out against the tube's own electronic noise. A dimensionless number.

How it is measured: at a standardised, genuinely dark light level of 108 microlux, over a circular reference area of 0.2 mm diameter and with 10 Hz bandwidth. The bandwidth is deliberately chosen to match the integration time of the human eye.

Why it counts: on genuinely dark nights the image is noise limited, not sharpness limited. SNR is then the value that decides between usable and unusable. For scale: one manufacturer measured that dropping from SNR 28 to 21, with all other values equal, costs 8 per cent of the detection distance under clear starlight.

If you are only allowed to read one line, read this one.

Limiting resolution (lp/mm)

What it is: the finest stripe pattern still resolved, in line pairs per millimetre, measured at the centre of the tube.

How it is measured: a person reads the highest still recognisable spatial frequency off a USAF test chart. That is as subjective as it sounds, and it is standard practice across the industry.

The trap: resolution depends strongly on light. That is why data sheets often carry a second resolution line at high illumination, typically at 200 lux. Two resolution figures on one sheet are not a contradiction, they are two different measurement conditions. Pay attention to which one you are reading.

What it does not say: limiting resolution is a single point at the threshold of perception. It says nothing about how much contrast survives on coarser structures, and that is exactly what decides whether an image looks clear or hazy. The value for that is called MTF and it practically never appears on a consumer data sheet.

EBI, equivalent background illumination

What it is: the faint self emission of the tube in complete darkness. It sets the lower limit of what the tube can render at all.

What you have to watch: the unit. This is where the most nonsense gets printed on data sheets. The European convention gives EBI in microlux. The US convention gives it in lumens per square centimetre, usually with an unspoken factor of 10 to the minus 11. The two convert into one another: 1.0 times 10 to the minus 11 lumens per square centimetre corresponds to 0.1 microlux.

We have seen dealer data sheets on which the exponent was simply left off, which falsifies the value by eleven orders of magnitude. An EBI figure without a unit, or without an exponent, is worthless.

And EBI depends on temperature. A warm tube glows more to itself. An EBI value without a measurement temperature is only half a statement.

Lower is better.

Halo

What it is: the ring of light around a bright point source. Given in millimetres.

Why it counts for more in daily use than almost anything else: street lamps, headlights, lit windows. Anyone who is not travelling exclusively in the middle of nowhere sees halo more often than they ever think about limiting resolution. Physically, halo corresponds to roughly four times the gap between photocathode and microchannel plate, so it is a property of the design.

What you have to watch: the size of the input light spot the measurement used. For the same tube family from the same manufacturer we have found two different halo figures in two different documents, because different measurement conventions sat behind them. A halo figure without its measurement condition is not comparable. Ask for it.

Smaller is better.

Photocathode sensitivity

What it is: how much current the photocathode delivers per incident luminous flux, in microamperes per lumen, measured against a light source of 2856 Kelvin, which simulates starlight.

Typical ranges: Gen 3 roughly 1350 to 2800, Gen 2 roughly 700 to 800.

The important qualification: higher is not automatically better in the finished device. On filmed Gen 3 tubes the ion barrier on the microchannel plate destroys a share of the photoelectrons. That is why a good tube with a markedly lower photocathode sensitivity can draw level with a Gen 3 on SNR. Read this line as an input quantity, not as a verdict.

Gain

What it is: the brightness factor of the tube.

The trap, and it is a large one: the USA and Europe measure at different input light levels, and because the gain ratio rises as input light falls, tubes measured to the European convention report structurally higher numbers for the same component. Two data sheets of different origin are not directly comparable on this line.

Note also that tube gain and the system gain of the finished device are two different things. System gain is markedly lower.

The blemish map

This is the part you really should look at, and often the only one with a graphic.

What it is: a map of cosmetic defects, dark spots, bright spots and chicken wire, sorted by count and by three concentric zones. Zone 1 is the centre, zone 3 the edge.

When a blemish counts: typically from about 0.076 mm diameter.

Usual grades: commercial goods typically allow up to four blemishes, one of them in zone 1. Military grades typically allow up to four, but none in zone 1 and at most three in zone 2. Aviation is stricter: none in zones 1 and 2.

Why the zone matters more than the count: three blemishes at the outer edge you will have stopped noticing after a week. One in the centre bothers you every single night.

A bare number without a zone therefore tells you almost nothing. Ask for the map, not for the counter.

Autogating

Often does not appear on the tube data sheet at all, because it is a property of the power supply and not of the tube.

What it does: it gates the voltage when bright light suddenly enters the scene, so that you are not dazzled and the tube takes no damage.

What you have to ask: present or not, and how fast recovery is. At the good end some tens of milliseconds, at the bad end around a second. One second of blindness when a vehicle comes round the corner is a practical difference that no number on the sheet captures.

And the Figure of Merit

It usually sits right at the top, and it is usually the only thing that was in the advertisement.

It is the product of SNR and limiting resolution, and if both values are on your sheet you can work it out yourself. Do that. It takes five seconds and it shows you which of the two values carried the high FOM.

If a tube reached its FOM mainly through resolution, it is optimised for conditions in which you do not particularly need the device. We have described that at greater length in a separate article.

The checklist

When you look at a data sheet, in this order:

1. Is there a serial number on it? If not, it is not a test report.
2. SNR. The single most important value for dark nights.
3. Blemish map, with zones. Zone 1 above all.
4. Halo in mm, with the measurement condition.
5. EBI, with unit and temperature.
6. Limiting resolution, and whether you are reading the dark or the bright light line.
7. Photocathode sensitivity, read as an input quantity.
8. Gain, in the knowledge that US and EU figures are not directly comparable.
9. Autogating, asked for separately if need be.
10. Work out the FOM yourself and see where it came from.

What a data sheet cannot do

It says nothing about the optics in front of it, nothing about the housing, nothing about the helmet mount and nothing about how the device behaves after three years. It is a snapshot of one tube on the day it was accepted.

That makes it the second best source of information. The best is a dark room and two devices side by side. That is exactly what the showroom in Villmergen is for, and you may bring your own device along for comparison.

If you have a data sheet and are not sure what is on it, send it to us. We will go through it with you, even if you did not buy the device from us. info@dtdsystems.ch, +41 76 225 50 04.

Related

• All DTD night vision and thermal devices
• Single tube monoculars
• Dual tube binoculars
• Look through one in Villmergen, by appointment

Every tube-bearing DTD device ships with the data sheet of the tube actually fitted in the box, on the DTD PVS-14 as on the DTD 1431 MkII, so you can read the real figures for your own unit exactly as described here.

Compare the tube grades from this post side by side: buy night vision in Switzerland.