What is a black and white transmission densitometer?
Densitometers are photoelectric measuring instruments that determine the gray value of black and white originals. Furthermore, they also measure the continuous tone or dot value. The most commonly used densitometers in printing factories are black and white densitometers. Generally, two types of densitometers are available:
- Transmission densitometer.
- Reflection densitometer.
The former type is appropriate for determining transparent originals, whereas the latter is used for measuring solid originals. The instrument’s principle is the evaluation of the degree of blackening of the photosensitive material after exposure and development. The range of a density meter is 0–2.5, and the higher the range, the higher the original’s blackness. If the blackness range is a significant number, the density is high and vice versa. The silver bromide on the photosensitive material is exposed to light during plate production and is converted to metallic silver during development, resulting in a degree of light resistance.
The device is primarily used to measure the density of liquids in physical experiments. It works on the principle that states that the buoyancy of an object floating in a fluid is equal to gravity. The density meter present in the instrument is a sealed tube made of glass with varying thickness and dense lead or mercury shot at the bottom. While using the device, the density meter should be positioned vertically into the liquid to be measured. Once the density meter reaches a stable state, note the density of the liquid displayed on the scale.
Working principle of digital black and white transmission densitometer:
Density is also known as optical density. The literal definition of optical density states “a value which is a physical quantity without a unit”. Optical density measurement is usually carried about by transmission black and white densitometer. As far as transmission density is concerned, it can be defined as the base-ten logarithm of the inverse Q of the transmission coefficient. Hence, a densitometer is an instrument used to indirectly determine the amount of light absorbed by a surface.
The instrument is also used to determine the amount of light passing through transparent materials to evaluate how much light is absorbed by the material surface. It consists of an illumination system, light collection and measurement system, and a signal processing system. The AC modulated light source works efficiently to provide a stable light source of a certain quality. The photoelectric sensor records the amount of light passing through the film under test. Ultimately, it converts the recorded amount into electric signals.
What is a black and white densitometer used for?
- As the film is exposed and developed conventionally, a negative is obtained with a variety of densities of multiple sizes. A densitometer can help regulate normal exposure and development.
- Both instruments are capable of measuring the density value and difference of the tested sample.
- Application in flaw detection: Radiographic flaw detection is widely used in industries. It is employed for metal and non-metal inspection. Radiography detects defect characteristic of internal structure, such as pores, pinholes, inclusions, porosity, cracks, segregations, etc., are seen by radiography. The method is primarily used for internal quality inspection of welds and castings such as boilers, pressure vessels, gas pipelines, automotive, etc.
How to choose a black and white densitometer?
Digital black and white densitometer
Black transmission densitometers are mainly employed for radiographic evaluation in the non-destructive testing industry. The digital black and white density meters are small, making them lightweight. The entire instrument has an integrated circuit and liquid crystal display. The size of the display panel is approximately 40mm x 15mm. The range for blackness measurement is 0–4D. The maximum measurement error is <±0.05, and a 9V stack battery is used as the power supply.
The portable forms of the instrument, when compared to the desktop version, come with fixed light sources and can determine highly accurate values from 40W bulbs to 8W fluorescent light sources.
Automatic zero return is a particular function found in the digital display black and white transmission densitometer. Generally, digital display instruments such as digital multimeters are enforced to return to zero through the short circuit of the needle. The function automatically returns to zero. Taking the logarithm of comparison with a reference light source yields the varied densities of the X-ray film or other films to be examined. The blackness display can automatically return to zero immediately during the dynamic rise and fall to prepare for the blackness value of the next point.
Transmission black and white densitometer:
The transmission black and white densitometer are primarily employed to evaluate the black and white density of X-ray film in medicine. The main features of the instruments include:
- Accurate readings
- Good Stability
- Easy operation.
The instrument comprises a light source, photoelectric probe, converter, amplifier, driver, synchronous detector, A/D converter, microcomputer, and display. When the instrument is turned on, the computer sends an AC signal to the driver, which ignites the light source. The converter changes the alternating signal received by the photodetector into direct current by an amplifier and a synchronous detector. The DC signal is converted by that A/D detector into a digital quantity processed by the computer. The density is calculated and displayed on the screen.
How to use a digital black and white densitometer:
Step 1: Switch on the power of the density meter. The display will present “— -,” which indicates that the instrument is functioning adequately. The instrument must be pre-heated for at least 5 minutes before proceeding with the measurement. Low charging can interfere with accuracy.
Step 2: Press the measuring arm and push the zero calibration button simultaneously. The display will show “0.00”; after that, release the zero calibration button and then the measuring arm. The instrument returns to its normal state.
Step 3: The sample should be placed in the measurement area, aligned with the light hole, and the measuring arm is pressed. The readings on display are the density value of the sample. The portable black and white transmission densitometer will determine the density value greater than 4.5D. As soon as the sample is detected, the instrument proceeds to enter the high-precision mode. The displayed value is the true value after stability is achieved.
Step 4: Release the pressing arm and observe that the displayed value remains constant.
Step 5: The density difference measurement includes measuring the reference sample after zero point calibration. Release the measuring arm once the reading is stable. Now, press the zero calibration key and wait for the display to show “0.00”. Place the competitive sample in the measurement area. The value displayed is the difference in density between the two samples.
Step 6: After zero point calibration, the instrument can enter the normal density value measuring condition without any sample.
Step 7: The instrument is based on two working states:
- Measurement Stage.
- Storage Stage.
The instrument proceeds toward the measuring stage when the measuring arm button is pushed. When the button is released, and the “Write” or “Read” button is pressed, the instrument will enter the storage stage.
Step 8: The “View| Delete” key is important in determining the stage of the instrument. If “Read” or “Write” is not displayed in the upper left corner, the device is said to be in a measurement state (“View | Delete” key is “View”), but when the “Read” or “Write” are displayed, the instrument is in a storage state (“view|delete” key is “delete”).
Step 9: The currently shown “density value” or “density difference” value can be stored when the measuring arm is released, and the “Write” key is pushed. The previously held values are displayed in order when the “Read” key is pressed.
How to calibrate a black and white densitometer?
Measured density values:
- Turn on the device and wait for the display to show “— -“‘. Pre-heat for 5 minutes.
- Press the measuring arm without any sample. The display will show “E.1”. Press the measuring arm button and the red zero adjustment button simultaneously. The window will display “0.00”. Release the measuring arm when the instrument is in the density value measurement stage.
- Align the test sample with the light hole, press the measuring arm, and the measured density result will appear in the display window.
- The zero point must be calibrated when using the standard density sheet. Align the reference point with the light hole without the reference sheet. Press the measuring arm along with the zero button. The display will show “0.00” and other measure points.
Observe the sample:
To see the test sample, turn on the lighting switch at the bottom left of the instrument and the LED lighting inside the device.
Serial calibration function:
Instrument Calibration is necessary when density sheets are not used. Calibration software can be used to calibrate the instrument and adapt to different density curves adequately.
What does D mean in a black and white densitometer?
The reflection density of a photosensitive material sample is the common logarithm of the inverse ratio of reflection flux to absolute reflection flux. The letter D stands for the density range, not the unit.