Measurement of Irradiance with a Fiber Optic Spectrometer

Irradiance Metrology is a branch of optical and radiometric metrology, primarily studying the distribution and variation of radiant energy on the surface of an object. One of its core concepts is irradiance, which represents the radiant flux received per unit area of the irradiated surface, measured in watts per square meter (W/m²). Irradiance is a physical quantity that describes how much radiant energy is received per unit area per unit time on a surface, which is crucial for understanding the transmission, conversion, and utilization of radiant energy.

Radiometric measurements can be used to measure both luminous sources and reflected light. For example, in solar spectrum research, it is used to obtain the energy distribution of the solar spectrum, measure the irradiance energy of the sun and the atmosphere, or the secondary irradiance reflected on objects. It is also used in LED light source parameter testing and quality inspection.

1. Principle of Irradiance Measurement with Fiber Optic Spectrometer

● Measurement of Test Objects: Irradiance measurement requires a standard light source with a known radiant energy distribution. Using a calibrated standard light source and a fiber optic spectrometer, the spectral radiant energy distribution curve of the test object can be measured. From this curve, parameters such as irradiance and radiant flux can be calculated.

● Measurement of Test Light Sources: Utilizing a spectrometer with spectral response characteristics to simulate the radiant energy distribution of the test light source, parameters such as irradiance and luminance can be calculated.

● Relative Radiometric Measurement: This involves comparing the radiation spectrum of the sample with the radiation spectrum of a standard lamp that has a black body radiation energy distribution (normalized to 1). The standard lamp is placed in the optical path first to measure its spectrum as a reference, then replaced by the test radiation source. This calibration only adjusts the peak shape and not the physical quantities, giving only relative intensity at 100%.

● Absolute Radiometric Measurement: A standard light source acts as an ideal gray body, using a tungsten-halogen lamp with a known spectral output (unit: µW/cm²/nm) to perform absolute radiometric calibration of the spectrometer. This calibration converts the spectrum collected by the spectrometer detector into the actual spectral distribution (shape and magnitude), changing the spectral intensity coordinates from relative intensity to actual irradiance intensity. By modifying the spectral shape and amplitude, the instrument's response function is corrected to measure absolute intensity.

2. Irradiance Measurement System

● Light Source Emission: Uses standard light sources with known irradiance (such as tungsten lamps, deuterium lamps) to emit stable spectra.

● Fiber Transmission: The light is transmitted through fiber optics to the spectrometer input. The fiber acts as a signal coupler, ensuring efficient transmission of the optical signal.

● Spectrometer Body: Comprising parts like the grating, slit, and detector. The grating disperses the light, the slit limits the incident light range, and the detector converts the optical signal into an electrical signal.

● Data Acquisition and Processing System: Responsible for receiving the electrical signals from the detector, amplifying, analog-to-digital conversion, and data processing, ultimately yielding the irradiance value.

● Optical Information Collection:

1) Cosine Corrector: Convenient for system setup, small collection module, wide collection area, cost-effective but requires a dark environment and has lower precision compared to integrating sphere tests, and is more susceptible to environmental fluctuations.

2) Integrating Sphere: Advantageous for testing with high anti-interference capability, internal dark environment, uniform illumination, accurate test data, small error, and high repeatability but is relatively expensive.