Reflection spectrum measurement

When light is incident on an object, there are two kinds of reflections, diffuse and specular. When a beam of parallel light strikes an object with smooth surface, the reflected rays are parallel to each other. This regular reflection is called specular reflection of light. For objects with rough surface, although the incident light is parallel, the reflected light is scattered in all directions. This phenomenon is the diffuse reflection of light.

Figure 1 Schematic diagram of specular reflection and diffuse reflection

When the object is in diffuse reflection, part of the incident light will produce specular reflection on the particle surface, and the other part of the incident light will refract into the surface particles to produce absorption, and the absorption intensity is related to the incident wavelength. Reflection spectrum refers to the change of the ratio of incident light intensity to reflected light intensity with wavelength.

When collecting the reflection spectrum, first measure the standard white board with known spectral reflectance to obtain its photocurrent spectrum, then:

Where is the spectral characteristic parameter of the system.

Then measure the sample to be tested to obtain its photocurrent spectrum, which also includes:

Divide the two formulas to obtain the spectral reflectance of the sample to be measured:

Through the above calculation process, we can see that the spectral characteristic parameters of the system (all effects produced by the test instrument) are eliminated in the last step, that is, they will not affect the measurement results. Even if the spectral characteristic parameters of the system are changed due to the influence of external factors, they will be completely eliminated in the measurement of the comparison method.

 Applications

Ø  Film thickness measurement

Ø  Color measurement

Ø  Meat sorting

Ø  Surface feature spectral measurement

Ø  Drug component analysis

Ø  Textile material testing

Ø  Quantitative analysis of moisture and sugar in agricultural products

Ø  …………

The reflectance spectrum measurement system used in this paper includes light source, optical fiber probe, spectrometer, high diffuse reflectance calibration plate and sampling accessories.

The adjustable tungsten halogen lamp can be used as the light source. The tungsten halogen lamp used in this paper is a desktop type optical fiber coupling output tungsten halogen lamp with low power consumption and high light output, and its wavelength range covers 360-2500 nm. The lamp has a service life of 6000 hours. The highly efficient and optimized optical design makes tungsten halogen lamps have the advantages of low power consumption and high brightness output. The optimized current stabilizing circuit ensures the stable spectral output of tungsten halogen lamp.

The principle of Y-type reflective optical fiber probe for measuring the reflectivity of objects is shown in Figure 1. The optical fiber adopts a Y-type structure. One end of two multi-mode optical fibers is combined to form an optical fiber probe. In the sensing system, one is a receiving optical fiber and the other is a light source optical fiber. The optical fiber only plays the role of transmitting signals. When the light emitted by the light source irradiates the reflector through the optical fiber, the reflected light is connected to the receiver through the receiving optical fiber to convert the optical signal into an electrical signal. The output light intensity depends on the distance between the reflector and the optical fiber probe, and the reflected spectrum is reflected through the detection of the light intensity. The use of Y-type reflective optical fiber probe for object sampling has the following advantages: simple operation, no damage to the sample to be measured, non-contact continuous measurement and so on.

Figure 2 Schematic diagram of Y-type near infrared optical fiber

The spectrometer is the core part of the reflectance measurement system. In the experiment, xsm11639 wide spectrum high-speed optical fiber spectrometer is used, with a spectral range of 200~1100 nm. It is equipped with double blazed grating and high-order filter. The high sensitivity, highly integrated circuit and compact structure design ensure the good communication speed and wavelength stability of the spectrometer, so that the working spectral range is not affected by materials. With the near-infrared quartz fiber, the optical signal is introduced by the fiber to isolate the instrument from the external environment, which can reduce the interference of the external environment to a certain extent.

The high diffuse reflection calibration plate, namely the standard white plate, is a calibrator for transmitting reflectance, which can be divided into transmission standard white plate, working standard white plate and reference white plate. Its technical requirements shall comply with the provisions of 5.1 and 5.3 in gb/t 9086-2007. For specular reflection samples, the specular reflection standard plate can be used as a reference.

Experimental example

1、 Experimental instruments

The instruments and equipment required for this experiment are shown in the table below:

MS11639 optical fiber spectrometer: Ms optical fiber spectrometer is an optical fiber spectrometer with a spectral range of 200nm-1100nm. The detector adopts Hamamatsu COMS detector. 16 bit a/d sampling and 75% quantum efficiency provide the spectrometer with high signal-to-noise ratio and large dynamic range. The spectrometer has good response performance in uv-vis-nis. Compared with common products, double blazed gratings are used to optimize the spectral response in UV and NIR bands, improve the sensitivity and efficiency of the spectrometer by 20%, and effectively reduce stray light by 50%. The ms11639 spectrometer equipped with double blazed gratings can effectively balance the full spectrum response, and can be widely used in physical and chemical analysis, biological samples, semiconductor material detection, optical detection, material detection and other fields.

Hl2000-adj continuously adjustable tungsten halogen lamp: hl2000-adj continuously adjustable tungsten halogen lamp is a desktop type low-power high light output fiber coupling output tungsten halogen lamp with a wavelength range of 360-2500 nm. The lamp has a service life of 6000 hours. The highly efficient and optimized optical design makes tungsten halogen lamps have the advantages of low power consumption and high brightness output. The optimized current stabilizing circuit ensures the stable spectral output of tungsten halogen lamp.

Qr600*7-vis-nir-2 near infrared quartz reflective fiber: 7-core composite design with core diameters of 105, 200, 400, 600, 1000 μ M optional, with bands ranging from deep ultraviolet to near infrared.

2、 Experimental design

2.1. instrument connection:

Connect the instrument as shown in Figure 1: fix the reflection probe on the measuring support, connect the six core end of the near-infrared quartz reflection fiber to the tungsten lamp light source, and connect the one core end to the spectrometer. Power on the tungsten lamp and connect the spectrometer to the computer with a USB cable.

Figure 3 Construction of reflectance measurement device

2.2. spectrum acquisition:

(1) Turn on the tungsten lamp to preheat for 3-5 minutes; Open the spectrum acquisition and analysis software, and select the reflection mode for the operation mode;

(2) The acquisition parameters are set as follows: the integration time is 200ms, and the integration times are 10;

(3) Save the dark background spectrum: insert the shading sheet into the light outlet of the tungsten lamp light source, click the "single" button to collect the dark spectrum, and save the dark spectrum after collection;

(4) Save the bright background spectrum: take out the shading sheet, place the diffuse reflection standard plate in front of the reflection probe, click the "single" button to collect the bright spectrum, and save the bright spectrum after collection;

(5) Place the standard color card on the stage under the Y-shaped reflection probe, and click "single" to collect the reflectance spectrum of the sample;

(6) After spectrum acquisition, click export data to save the data, and read out the relevant light color parameters, color coordinates XYZ, tristimulus XYZ, lab, sunlight reflectance, near-infrared reflectance and other data from the right side of the software interface. The specific operation interface is shown in the following figure.

Figure 4 Uspectralplus software color measurement mode interface

3、 Experimental results

The measured reflectance spectra of 13 samples are shown in Figure 3:

Figure 5 Reflectance spectra of different color cloth samples