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White vs Green

One of the most essential parts of an IIT is its phosphor screen. It is what you see and what converts the electrons back to photons.

Typically, the phosphor is deposited on the back of the fiber optic and then covered with a layer of aluminum. The aluminum layer increases the light efficiency up to almost 100% by reflecting the light and reducing the electrical charge retained.

There is a multitude of phosphor types that are usually referred with with a P-number i.e. P-22. Four parameters are the most critical ones for evaluating a phosphor screens performance and suitability: Conversion factor, light spectrum, decay time and resolution.

Conversion factor
Refers to conversion rate of electrons vs photons. So how many photons are emitted by one electron hitting the screen.
A high conversion rate does not necessarily mean the eye will perceive a bright image as that depends entirely on the emitted light’s wave lengths.

Light spectrum
The emitted light spectrum of the screen i.e. 475nm-600nm for P43.

Decay time
The time the screen needs to go back to a low brightness after being activated. It has to be multiple times shorter than the eyes temporal inertia. It is comparable to the refresh rate of a TV screen or a light bulb.

Resolution
This is depending on the size of the average size of particles that are the screen. Bigger particles would be more efficient but would eventually create a literally grainy image. Normally this is kept at about twice the resolution of the final system.

As a fifth indicator serves the luminous efficiency. This is not a fixed number but rather describes how well a phosphor screen produces light within the visible light spectrum or in other words how well the light output of the screen fits the to the human eye.

The different phosphors

Today (with very few exceptions) tubes are built using either P22, P43 or P45 phosphor. There are arguments for and against each of them and this will be a list of some of them.
In the end someone has to try what suits for their purpose and eyes.


Note: Picture of P20 (very similar to P22) and P45 taken with day cap (pin hole), P43 with full daylight filter

P22 (green)

Pro:

  • High luminous efficiency, almost the same as the human eye for the color green
  • Less visible noise
  • Human eye is most sensitive to contrasts with green colors
  • Very suitable for static observation

Con:

  • High decay time (60ms to 10%)
  • Faster eye fatigue due to bright green image

 

P43 (yellow-green)

Pro:

  • High luminous efficiency, almost the same as the human eye for the color green
  • Human eye is most sensitive to contrast with green colors
  • Low decay time (1.2 ms to 10%)
  • More yellow-green image compared to P22 and less eye fatigue in comparison
  • Much higher conversion rate and light emissions compared to P45

Con:

  • Faster eye fatigue compared to P45
  • Narrower bandwidth which uses basically just one type of receptors (cones) in the eye where P45 uses most of the bandwidth

 

P45 (white)

Pro:

  • White light feels more natural in low light environments and therefor causing less eye fatigue and having a noticeably perceived brighter image
  • As recently introduced into (broader) production, most of these tubes use state of the art technology
  • Low decay time (1.5ms to 10%)
  • White light allows to use the rod cells in the eye which are most sensitive in low light which can create a brighter perceived image

Con:

  • Lower luminous efficiency than P22/P43 which can produce a darker perceived image
  • Hardly any eye pieces match the bandwidth of P45 which can cause distortion on the image (chromatic aberration) and hard to design for the wide bandwidth

 

Summary

There are strong arguments between green and white phosphors and both have their up and downsides. A noticeable trend from green to white is visible in the market, especially the US market and since P45 tubes are now certified for aviation this will increase.
As the broader introduction of P45 tubes is fairly recent to this date, the performance and technology is absolute state of the art like filmless or 4G IITs which enhances the performance compared to older P43/P22 tubes.
As everyone is different and their eyes are different, you cannot say that P45 or P43 is better as it rather depends on the users and what they can work better with.

Provided by ActinBlack