Photometry:
The measurement of light as perceived by human eye is called photometry. Photometry is measurement of a physiological phenomenon, being the stimulus of light as received by the human eye, transmitted by the optic nerves and analysed by the brain. The main physical quantities in photometry are (i) the luminous intensity of the source, (ii) the luminous flux or flow of light from the source and (iii) illuminance of the surface. The SI unit of luminous intensity (I) is candela (cd). The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency `540 xx 10^12` Hz and that has a radiant intensity in that direction of 1/683 watt per steradian. If a light source emits one candela of luminous intensity into a solid angle of one steradian, the total luminous flux emitted solid angle is one lumen (1m). A standa o incandescent light bulb emits approximately 1700 lumens.
What is photometry?

Options

A. Measurement of light as perceived by human eye
B. Measurement of number of photons emerging from a light source
C. Measurement of electrons emitted by photosensitive surface
D. Measurement of photosensitivity

Explanation

### Step-by-Step Solution:

1. **Understanding Photometry**: Photometry is defined as the measurement of light as perceived by the human eye. It involves the physiological response of the eye to light stimuli.

2. **Process of Light Perception**: The process begins when light enters the human eye. The eye acts as a receptor, receiving the light signals.

3. **Transmission of Signals**: Once the light is received by the eye, the signals are transmitted via the optic nerves to the brain.

4. **Analysis by the Brain**: The brain then analyzes these signals, allowing us to perceive and interpret the light.

5. **Key Quantities in Photometry**: Photometry involves three main physical quantities:
- **Luminous Intensity (I)**: The amount of light emitted by a source in a particular direction, measured in candelas (cd).
- **Luminous Flux**: The total amount of light emitted from a source, measured in lumens (lm).
- **Illuminance**: The amount of luminous flux per unit area on a surface.

6. **Units of Measurement**: The SI unit of luminous intensity is the candela (cd), and one lumen is defined as the luminous flux emitted into a solid angle of one steradian by a source with a luminous intensity of one candela.

7. **Conclusion**: Therefore, photometry is fundamentally concerned with how light is perceived by the human eye, the physiological process involved, and the quantification of light in terms of intensity, flux, and illuminance.

### Final Answer:
Photometry is the measurement of light as perceived by the human eye, involving the reception of light by the eye, transmission of signals through the optic nerves, and analysis by the brain.

---