A luminance control circuit has luminance change characteristics similar to those
of a conventional lamp when a light emitting diode is used as a light source. The
luminance control circuit adjusts the value of a forward current flowing in the
light emitting diode in accordance with a control voltage which is obtained by
smoothing a light adjustment pulse signal from an illuminance control circuit.
The circuit has: a pulse adjustor for adjusting a duty factor of the light adjustment
pulse signal according to characteristics of the light emitting diode; holding
circuit for holding the control voltage to a predetermined value or more; and a
switch for interrupting the forward current flowing in the light emitting diode
by using the light adjustment pulse signal or a pulse signal after the adjustment
of the duty factor, wherein at least two of these three elements are used in combination.