In this paper, the methods for determining the opacity and transparency of paper are described, and the reflectance factor, internal reflection factor, and opacity of paper are defined. The required instrument (reflectophotometer, standard whiteboard, standard black cylinder), the preparation steps of the specimen, and the calibration procedure of the instrument are detailed. At the same time, the operation steps in the measurement process and the calculation method of the results are introduced to ensure the accurate evaluation of the optical properties of the paper.
Purpose and Scope of Application
Objective: The purpose of this method is to determine the reflectance factor, internal reflection factor, and opacity of paper to evaluate its optical properties and applicability.
Scope of application- Suitable for inspecting near-edge colored papers, whether they contain optical brighteners or not, to ensure accurate measurement of the optical properties of different types of papers.
definition
The reflectance factor (R) is the ratio of the radiant flux reflected by an object under a particular lighting condition to the radiant flux reflected by a fully reflective body under the same conditions, usually expressed as a percentage. This value reflects the overall light reflection capacity of the paper.
Reflection Factor (R) The reflection factor value of the paper pattern when a single layer of paper pattern is backed by a standard black cylinder. This value is used to measure the ability of a paper to reflect light in the absence of light transmission.
Internal reflection factor (R) The reflection factor measured when the thickness of the paper pattern increases to a certain extent that the reflection factor no longer changes significantly with the increase in thickness. This value represents the intrinsic optical properties of the pattern.
Opacity of paper – calculated by the ratio of the reflectance factor (R) of a single-layer pattern with a black backing to the internal reflection factor (R internal reflection) of the pattern. Indicates the ability of the paper to cover light and is usually reported as a percentage.
Instruments & Equipment
Reflectance photometer – used to measure the light reflectance factor of paper under specific conditions. The instrument should have high accuracy and stability to ensure the accuracy of the measurement results.
Working Standard Whiteboard – Used to calibrate the reflectance photometer and ensure accurate readings from the instrument. The whiteboard should have a known and stable reflectance factor.
Standard black cylinder - used to measure the reflectance factor of the paper sample, its reflectance factor should not be greater than 0.5%. The function of the black cylinder is to provide a standard optical background in order to accurately measure the reflected light properties of the paper.
Preparation of specimens
Samples were taken from paper according to standard methods to ensure sample representativeness and reliability of the test.
Cut at least 10 100mm x 100mm square specimens from the sheet. One side of each specimen should be aligned longitudinally with the paper and stacked upwards. Ensure that the total thickness of the stack meets the requirements of a stable reflectance factor, that is, the reflectance factor does not change significantly with the increase of the number of layers of the specimen. A test piece is placed on top of the stack to protect the specimen from contamination or light during handling.
Calibration of the instrument
According to the instructions of the instrument manual, install the appropriate filters into the incident beam and the reflected beam respectively, and then turn on the power switch of the instrument.
After the instrument has warmed up to the specified time, the instrument is calibrated using a working standard whiteboard to ensure the accuracy of its readings. Subsequently, a standard black cartridge is used for further calibration to ensure the reliability and consistency of the measurements.
Assay procedure
Remove the protective layer from the stack and test the internal reflection factor (R internal reflection) of the topmost specimen with an accuracy of 0.1%. Remove the specimen and place it at the bottom of the stack, repeat the test of the second specimen and continue to test at least 5 specimen in the same way. After flipping the paper, repeat the above operation to determine the internal reflection factor (R internal reflection) on the reverse side of the specimen.
For specimens that have been measured with an internal reflectance factor (R), a single-layer backing standard black cylinder is tested, and the reflectance factor (R) of the front and back sides of the specimen is measured separately.
Calculation of the results
method
First, the average of the reflectance factor (R) and the internal reflection factor (R internal reflection) of all test data on the front and back sides of the specimen are calculated separately.
Use these averages to calculate the opacity of heads and tails by formula.
If the difference between the positive and negative measurements is greater than 0.5%, the opacity and transparency values of the front and back should be reported, respectively; If the difference is less than 0.5%, the average of the heads and tails is reported and the results are accurate to 0.5%.
Note: For each piece to calculate the opacity individually and to calculate the opacity using the average of all the pieces, the difference in results is small, and the latter is recommended as a calculation method to simplify processing.
summary
Through the method in this paper, the opacity and transparency of paper can be accurately determined, which provides a scientific basis for the quality control and application of paper. Critical steps include precise specimen preparation, proper calibration of the instrument, and detailed measurement procedures. When calculating the results, special attention should be paid to the difference between the positive and negative data, and the appropriate calculation method should be selected to ensure the reliability of the data.
