Microscope Activities, 1: The Light Source

In the past, Hooke College of Applied Sciences offered a microscopy workshop for middle school and high school science teachers. We thought that these basic microscope techniques would be of interest not only for science teachers, but also for homeschoolers and amateur microscopists. The activities were originally designed for a Boreal/Motic monocular microscope, but the Discussion and Task sections are transferable to most microscopes. You may complete these 36 activities in consecutive order as presented in the original classroom workshop, or skip around to those you find interesting or helpful. We hope you will find these online microscope activities valuable.

EXPERIMENT 1: Illumination: Lighting the Specimen for Transmitted Brightfield

Goal

To become familiar with the importance and control of specimen lighting.

Level

Basic

Materials Needed

Microscope

Procedure

Turn the microscope completely around from its operating position, so that you can see both the bottom rear of the microscope and the right side (left side in rear view), as shown in the Figure 1-1.

Discussion

The ON/OFF control is a rocker switch labeled “MAIN”, followed by the international symbol (l/O) for ON and OFF, which may be thought of as binary designations l (one = ON) and 0 (zero = OFF). Pushing this rocker switch to the ON position allows electricity from the built-in rechargeable battery to flow to the white LED (light emitting diode) that serves as the microscope’s fixed light source.

Figure 1-2 shows the AA 1.2V 1500mAh rechargeable battery pack that is visible on the inside of the hinged base plate after the locking screw is unscrewed; the leads going to the LED may also be seen.

The brightness level of the light is controlled by turning the knurled wheel of the rheostat, seen to the far left in Figure 1-1 (right side from operator’s position). As indicated by the symbol, rotating the knurled wheel to the rear (high side of the black incline symbol) increases the intensity of the light; rotating the wheel toward the front of the microscope decreases the light level. With LED illumination, changing the brightness does not appear to change the color temperature (“whiteness”), as happens so dramatically when using incandescent bulbs for illumination. When using incandescent bulbs for microscope illumination, it is important for critical microscopy and photomicrography to operate the bulb at the voltage specified by the bulb manufacturer, e.g. 12V. At this voltage, the light will be of a specified color temperature – typically 3200K for quartz-halogen bulbs. Then a blue filter (typically one designated 80A) is placed in the light path to convert the color temperature from 3200K to 5500K (photographic Daylight). Once the correct daylight color temperature is achieved, intensity of the light should only be controlled with a series of neutral density filters (e.g., ND 0.3, ND 0.6, ND 0.9, ND 1.2).

The LED is said to provide light for the microscope for up to 80 hours without being plugged into the wall socket. The built-in rechargeable battery can be recharged by plugging in the supplied recharger. In Figure 1-1, the connecting plug from the recharging unit is shown just in front of the receptacle where it is plugged in; the symbol indicates the center positive terminal, and the outer negative terminal; the other end of the recharging unit is plugged into the wall socket. The LED in the microscope is rated at 3.5V 120mW max; the recharger supplies OUTPUT DC 4.5V 1A.

To the far right in Figure 1-1, a standard USB connector, with international symbol, can be seen. This connecting port is used for connecting the built-in digital camera to a computer or laptop when using the optional software.

Task

Connect the recharger, if necessary, to the center receptacle. Connect a computer or laptop to the microscope using a suitable cord with USB connectors. Push the rocker switch to ON. Rotate the rheostat to adjust the brightness of the LED to a comfortable level.