Focused Practice

I. Answer the following questions:

1. What categories do analog analyzers fit into?

2. What do network analyzers characterize?

3. What do signal analyzers characterize?

4. How many instrument types do signal analyzers include?

5. Are spectrum analyzers among the instruments in the signal-analyzer category?

6. How many ports can the networks have?

7. What do nonlinear components usually require?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: Network analyzers.

Unit 55 Grammar: The Passive Voice.The Gerund Word List:

1. aperture	апертура, эффективная площадь отверстия, через которое проходит световой поток
2. pinhole	отверстие, размером с иголку
3. optical train	ряд оптических преобразователей в микроскопе
4. vibrating tuning fork	сканирующий настроечный разветвитель
5. wide field microscope	широкоформатный микроскоп
6. point detector	точечный детектор
7. PMT-photomultiplier tube	фотоумножитель
8. avalanche photodiode (APD)	лавинный фотодиод
9. noise-free amplifier	малошумящий усилитель
10. signal-to-noise ratio	отношение сигнал-шум
11. image plane	фокальная плоскость
12. resolution	разрешающая способность (различимость)
13. sample stage	обозреваемый участок образца

Confocal Microscopes

In 1957, Marvin Minsky, first proposed using an aperture that was optically conjugate to both the source and the focus spot on the sample, making all three planes mutually in focus. A second optically conjugate pinhole was also used in the optical detection train. To obtain an image, the sample stage was scanned in the X and Y directions by vibrating tuning forks.

Such a microscope has two advantages. The detection aperture blocks the light that originates from outside the image plane. This enhances the contrast, enables three-dimensional image acquisition, and improves resolution. With small enough pinholes, the resolution can be improved by 40% compared to the Abbe limit for a conventional wide field microscope.

About a decade after Minsky, Mojmir Petran and his coworkers at Charles University developed a confocal microscope based on the Nipkow disk. In a Nipkow disk, a series of holes arranged in a spiral are used as the aperture for the source. A complementary spiral of holes on the other side of the disk is used for detection. The image is scanned by rotating the disk at high speed.

In a scanning confocal microscope a point detector such as a PMT, avalanche photodiode (APD), or photodiode are required.

Photomultiplier tube (PMT) is not only an efficient detector, but a noise-free amplifier as well.

In low light levels in all of the confocal designs, the detectors play a very important role in determining the quality of the image. Image quality is directly related to the signal-to-noise ratio (S/N) of the detected optical signal. The S/N is determined by the amount of light that reaches the detector, the noise generated in the detector, and the detection electronics.

The optical signal that reaches the detector will be determined by the light source, the optical train of the microscope, and the sample. For biological samples, the total optical energy that can be imaged onto the sample is limited by damage to the sample or to the fluorescent probe.