Backlighting panel LCD

Light spread is much more even when seen with the eye. A camera
tends to make the area around the LED look brighter.

Overall brightness is more like around the three left-hand digits

Flaring by the camera does have its upside. The LED can be manoeuvered
looking through the viewfinder until the best evenness is observed.
With careful placement, good average illumination is quite doable The diffuser is simply a piece of 2mm acrylic the size of the underside of the glass. It's covered with adhesive aluminium foil except for the top face and one or both ends. Alternatively all edges can be covered and a gap left for the LED to shine into. Interestingly, flaws and chips on the edges translate to slightly different illumination. If the front face were scuffed or sanded this may produce a more uniform brightness A small dot (3mm square) of foil on the very end of the white LED helps to break up the beam (LCD is not seated firmly in a socket so RH digit is incomplete)

This alternative diffuser is even better. Light distribution is much
more uniform to the naked eye, as evidenced by using a contrast filter
on the camera. With some minor refinements there should be very little
gradient across the display. There's a noticeable increase in brightness
on the left-hand side when the left-hand gap is covered with foil. Light
from the sole LED (on the right) reflects back into the diffuser

"white" and "hot pink" were done using 2 x 3mm white LEDs, each with 100R
and a 500R trimpot in series. Brightness is good and well spread. White
paint gives a more even and less blue display than aluminium foil, but
requires 2 LEDs. All others were 2 x 5mm white LED with 100R in series
Rub-on letters and graphics are quite readable through the display, but
being slightly fainter than the segments, do not draw attention away from
the digits

Inverse display
(camera flaring not corrected)