Light Source and Illumination System of Microscopes
The light source and illumination system delivers light to illuminate the specimen. Microscopes employ two categories of light sources: natural light and electric light. For microscopes running on natural light, the light source system consists of only a reflecting mirror (also referred to as a mirror). It is mounted on the microscope arm below the condenser. The reflecting mirror has two reflective surfaces - one plane and one concave - and can be rotated freely in both horizontal and vertical directions. Its main function is to redirect ambient indoor light toward the condenser; the concave surface also provides a degree of light-gathering effect.
Most modern microscopes use electric light sources for illumination. The light source system is composed of the lamp circuit, light source lamp, lenses, reflecting mirrors and condenser, with the entire illumination assembly housed inside the lamp holder. Tungsten lamps or tungsten-halogen lamps are commonly adopted as light sources, with power ratings ranging from approximately ten to several tens of watts. The lamps typically operate at a low voltage of 12 V or below, and require adjustable voltage to vary light brightness. A brightness regulator is usually built into the lamp circuit, enabling convenient adjustment of the light intensity projected onto the specimen.
Lamp circuits are available in a wide variety of structural configurations, but voltage regulation is generally implemented by adjusting the voltage across the primary winding of the transformer. Common methods for adjusting the primary winding voltage include: stepping the primary voltage via tapped series resistors connected to the transformer primary; and varying the conduction angle of a silicon controlled rectifier (SCR) using a unijunction transistor or diac to modify the transformer primary voltage.
