One is laid bare by [WizardTim], as he cross-sections a 20KV high-voltage diode. A conventional low-voltage silicon diode has a forward voltage drop of about 0.7V and a relatively low maximum ...

Then knowing he needed 4 mA to power each LED for backlighting he used some 1978 vintage current limiting diodes designed to pass 2mA each regardless of voltage source, within limits of course.