Back-lit switches
Tek introduced back-lit pushbutton switches at the end of the 1960s in the 7000 scope family.
There is one miniature 5 V incandescent lamp per interlocking group of buttons, with clear plastic light guides reflecting the light to the backs of the transparent plastic buttons. In the Off state, a small metal flap covers the un-lit button end, in the On state it is pushed open.
The lamps are not supplied from the standard +5 V (logic supply) rail but from a separate lamp supply rail which is not present in all mainframes (e.g. missing from most 76x3 series scopes). In mainframes that do supply the lamps, the lamp voltage (and therefore lamp intensity) can often be varied, e.g. through a 3-way (off/mid/high) switch in the back of the instrument. In the 7904 (non-A), this is a front panel control.