A colleague asked me whether the trailing effect you see on the seconds hand was a photography trick, or something actually visible to the human eye.

Trailing effect

Yes, the trailing effect is real, and it’s thanks to a characteristic of the CRT’s phosphor screen called persistence.

Think of the watches with hands that glow in the dark – they absorb light, and gradually emit the light when it is dark. The phosphor coating in the CRT is the same – even after the electron beam moves away, the excited phosphor persists the light for some time.

How long the persistence, and what color, depends on the type of phosphor compound used in the CRT. For example, my Prototype model has a beautiful blue trace, with a medium-length green persistence. You can see this in the below slow-motion capture.

Beautiful blue and glorious green

On the other hand, the CRT used in the Model 1 has a bright green trace, with a relatively shorter persistence that is also green. So the ‘trailing effect’ isn’t as noticeable in that CRT.

Types of CRT phosphors

Types of phosphors were standardized from around WWII, and in the American scheme were represented in the CRT name by ‘P’ followed by a number. For example, 3BP1 uses a P1 phosphor.

I consulted my oscilloscope bible, Encyclopedia on Cathode Ray Oscilloscopes and Their Uses (a 1950 John F. Rider book). Already by that time, there were several standard phosphors with different trace and persistence characteristics.

In the last 60 years, many more phosphor compounds have been brought to light. Visit Wikipedia – Phosphors for a sickening amount of detail on this topic. If you are still unenlightened and you are extremely persistent, purchase Practical Applications of Phosphors and try mixing your own phosphor compounds, for use in your own hand-made CRT.

Long-persistence radar screens

Vintage radar equipment is very cool. The screens made use of P7 or P12 type phosphors to persist the image as long as possible, while the beam rotated around.

Looking around, I see at least one example of a scope clock that uses a radar CRT – quite an impressive display!