This is probably the question we get asked the most: If Echos are run-offs of the Photonis INTENS 4G line due to factory dark spots (aka blemishes), how many dark spots should I expect and how bad are they?
Let's call a spade a spade right off the bat and be honest - Echos are higher-performing and more affordable than any other comparable image intensifier tube available, but yes, they have factory dark spots in the form of small dots in the image. Second - while Echos initially were 4G run-offs, this is no longer the case and Photonis is now making these to fill the product line. Third - factory dark spots and blemishes are two different things. Factory dark spots are as a result of any number of things, typically a piece of dust trapped in between the MCP and photocathode plate, as an example, or some other by-product of the manufacturing process. Blemishes are user-induced tube damage like laser burns, leaving a weapon-mounted monocular behind a red dot on high, etc.
Rather than try to describe the dark spot tolerances, we thought we would show you via the standard NSN 5855-01-548-9489 spot chart.
This chart conveniently divides the viewable area of any tube into three zones: 1, 2, and 3. Zone 1 is probably your most critical zone and likely where your eyes will naturally focus. You will likely find yourself centering your head to whatever you're aiming so that your target falls within this zone. Notice however that Zone 1 is actually quite small in comparison to Zone 2. Zone 2 will be where you'll likely pick up surrounding information to form "the bigger picture". Lastly, Zone 3 is a very narrow band on the outer edges. Typically this zone, while somewhat useful, suffers from slight edge distortion and light fall-off (vignetting) from optics. This is what we would consider the periphery and the limits of detectable information, and probably the least critical of the three zones.
Next, moving onto the spot sizes, the chart conveniently provides reference blemishes located above the Zone 1 ring, starting with 0.003" and going up by 0.003" increments up to 0.015".
Using a representative sample size of 50 Echo tubes, we're including a through-tube photo of what we would call the "worst" out of the batch. You can click on the image for a full-resolution version (opens a new window).
Working our way through the zones, there is one Zone 1 spot that's roughly 0.003" and two smaller ones that - because they are smaller than 0.003" are not considered spots; two Zone 2 spot, one at 0.003" and another at 0.006" in size; and finally three 0.003" spots in Zone 3.
On the opposite end of the spectrum, within that same batch, here is a tube photo of the "best" tube.
There is a single Zone 3 spot that is 0.003" or less in size.
Bear in mind that the above represents a random sample of 50 tubes. Formally, Echo tubes have the following spot tolerances. Any tube with more spots than these specs are automatically rejected and destroyed by Photonis. Note that the existence of any spots above 0.012" in size are automatically rejected also.
| Spot Size | Zone 1 | Zone 2 | Zone 3 |
| 0.009" - 0.012" | 0 | 1 | 2 |
| 0.006" - 0.009" | 1 | 2 | 4 |
| 0.003" - 0.006" | 2 | 4 | 6 |
There may be instances where there are more spots of smaller size than is allowable per zone. For example: three 0.003" spots in Zone 1 are allowable.
Finally, fixed-pattern noise such as honeycombs (typically most obvious during high light situations) and slight shading are not considered blemishes.
All of our systems are built with these tubes described above and will be drawn at random for builds. We will work with you to determine eye dominance and place the tube with the cleanest image over your dominant eye.
We capture each and every tube image at the time of build completion and store it in our database with its respective serial number and spec sheet.