Photometer

Photometers are basically used to measure illuminance. Based on the unit of illuminance, the photometers that are only used for this purpose are also called luxmeters. However, the photometers can also be used to determine the proportion of foreign substances or the concentration of the known mixtures of substances via the attenuation the light intensity. These instruments, which are equipped with special sample containers, are also called turbidity meters. The selection criteria for a photometer depend primarily on whether it is to be used for measuring the illuminance or as a measuring instrument for determining the properties of liquids.

Selection criteria for a photometer for measuring illuminance
For the human eye, capturing details, colors and spatial structures is more difficult in increasing darkness. Situation-specific lighting is therefore important for recognizing dangerous situations and increases the ability to concentrate on work and learning tasks. This applies equally to stay in private and public areas. Light also plays a decisive role in the development and growth of plants and the durability of certain substances. In occupational safety guidelines, accident prevention regulations and technical standards, a minimum brightness value is prescribed or recommended for many areas. Compliance with these values for illuminance is measured in lux at the level relevant to the use, for desks or workbenches, which means on the table surface, for wall boards on the board surface and for traffic routes on their surface. The illuminance is a surface-dependent value that indicates how much luminous flux is incident on a surface. One lux corresponds to one lumen per square meter.

The measuring range of the photometer
In clear weather, natural daylight reaches much higher values than required for visual tasks and most technical and natural processes. For the evaluation of the necessary measuring range it is important to clarify whether only minimum values should be checked with the photometer or whether high daylight values must also be recorded for certain evaluations. For indoor measurements, a measuring range of 2,000 lux or 10,000 lux is generally sufficient; for outdoor daylight measurements, the measuring range should be 100,000 lux or higher.

Photometer accuracy
The measuring accuracy of many devices is 3 or 5 percent of the measured value. If these measurement deviations have a strong effect on your application, it is better to choose a device with the better resolution. In addition, an attention should be paid that some photometers with their sensor and the corresponding filter are designed for certain light sources. These devices can measure the illuminance for these types of light more accurately than those from other light sources. For the measurements of the illumination of living and working areas, the spectral sensitivity of the photodiode should be adapted to the brightness sensitivity of the human eye. For this purpose, the International Commission on Illumination (CIE) defined the light efficiencies of different wavelengths for the eye of a normal observer adapted to bright and dark.

Measurement of light colors
Some photometers for illumination measurement do not only register the illuminance but also the color coordinates of the incident light. The colorimetric values can be specified as three primary colors in three-part additive color models. The values are then specified, for example, as X, Y and Z values according to the CIE-XYZ color space or as red, green and blue values according to the CIE-RGB color space. Many of these photometers not only display numerical values for the respective primary colors. Often they show the color distribution also graphically on a color display.

Photometers for mobile or stationary measurements
Checking the lighting of workplaces and traffic routes is usually done with mobile devices. In the areas where too little or too much light can cause serious damage, or where a limited range of illuminance leads to optimal results, the use of stationary photometers is recommended. Both mobile and stationary photometers are available with continuous measurement function and data storage. Stationary devices often offer the possibility to control the systems for shading or artificial light sources or to trigger optical or acoustic warnings via relay or control outputs when certain values are reached.

Selection criteria for photometers for turbidity measurement and analysis of liquids
The extent to which a medium obstructs the passage of light can be determined with the photometer. For this purpose, the illuminance without weakening by the substance is compared with the illuminance after penetration of the medium. One application is, for example, testing the light transmission of glasses and transparent plastics for light of certain wavelengths. These values are important, among others, for ophthalmic lenses, technical glasses and glazing on residential and industrial buildings. For such measurements, the photometer should be designed for the wavelengths to be filtered or allow the adjustment of different wavelength ranges. In the case of light-transmitting liquids, the value of the attenuation of light is often directly related to other properties of the substance mixture.

Measuring methods and measured values in a photometer for turbidity measurement
The photometer uses a light sensor to measure optically how strongly the light that penetrates a substance is attenuated. In order to reduce the influence of color-dependent absorptions in colored substances, a light source for infrared light in the wavelength range around 860 nanometers is often used. The emitter and receiver of the light can be arranged in the photometer in different ways. In all arrangements, the photocell measures the intensity of the light, both with and without the substance under investigation, to take into account the influence of the sample container.

Transmitted light measurement – 0° method
The measurement of the radiation which penetrates the sample in a straight line is professionally defined as turbidimetry. This method is well suited for strongly turbid substances. Light source and light sensor are positioned on opposite sides of the sample. The measured values are usually given as formazin attenuation units (FAU) according to EN 7027-1. The values between 40 and 4,000 FAU are common.

Measurement of lateral scattered light – 90° method
The method in which the light scattered laterally by the sample is detected is called nephelometry. This measurement is often used for the substances that are only slightly turbid. The receiver unit is arranged at right angles to the light source and detects only the laterally scattered light. According to EN 7027-1, the value is given in the unit NTU (nephelometric turbidity unit). The values are usually between 0.05 and 400 NTU. Special units are available, for example, for measuring turbidity in beer with EBC in Europe and ASBC in the USA.

Backscattered light measurement – 180° method and multi-angle measurements
For various applications, the light reflected back in the direction of the radiation source is also measured. For these measurements, the transmitter and receiver units are placed directly next to each other. Some photometers are also equipped with several sensors. This allows for example the measurement of transmitted light and 90° sideways scattered light in one device. Depending on the intensity of the turbidity, the appropriate one is then automatically selected from the two measured values. In other devices, different sideways scattered light is measured simultaneously and evaluated together.

Data storage and transfer in the photometer
Photometers with data memory simplify the performance of measurements and minimize reading and transmission errors. Especially when measuring directly at the sample location it is useful to store the data in the instrument. If a lot of measured values should be stored, it is essential to pay attention to the memory size and the comfortable retrieval and transferability of the data. Depending on the equipment, the measured values can be transferred to a PC or smartphone via USB cable or Bluetooth, for example.

Analysis of ingredients with the photometer
Since the turbidity increases with the concentration of the substances dissolved in the liquid, photometers can be used to determine the concentration. If it is certain that in the sample under investigation a liquid of a certain concentration is free of impurities, the concentration can be derived directly from the turbidity value via the calibration curves. If particle sizes are difficult to determine or if various impurities are present, the property of these substances to react with certain chemicals is used to measure certain components. For this purpose, the reagents are used that only in reaction with the substance to be measured lead to the production of characteristic additional turbidity. The end products of these reactions can then be recorded and evaluated in a substance-specific manner. Certain values must be stored in the photometer for these measurements. If you intend to use a photometer for such analyses, you should choose a device that is already designed for the measured variables. In addition, you should pay attention to which measuring accessories are available, how the measuring process is performed and whether and how the photometer can be calibrated.

Water is life – an old statement that no one can disagree with. However, a lot of attention should be paid not only to the amount of water available, and its distribution, but also to its quality. Water is around and inside of the human beings; it is in the atmosphere, on the surface of Earth, under the earth, it has different states (from hard to liquid) and the first association that people get when asked to describe water in simple words, is that it has no color, no taste, no smell and that it is liquid. In the modern society water is taken for granted, not any single day can be imagined without water: it is running out of the tab, it is sold in the bottles, it is used in absolutely all industrial processes, in agriculture, and one of the sad and alarming consequences of that is that water is treated badly, contaminated by very harmful substances, wasted in excessive amounts and there are not enough purifying facilities.

Many people know that water is hard or soft, without giving a thought what it is and why it may be different. Hardness is in fact a property of water, which is determined by the presence of calcium and magnesium in it. In fact, it is a quality characteristic and it plays a huge role in assessment of the quality of water supplied for households, industry, technical needs. These elements, calcium and magnesium are not the only ones that are contained in the water, but particularly these two values influence the hardness of water the most. They are known as important for the organisms and from this side, certain amounts of calcium and magnesium in the water used for consumption are even beneficial. Teeth, bones, cells in the body – these parts in the human body do need auxiliary elements, like calcium and magnesium, for firmness and healthy functioning and thus, water becomes a good source of them.

From the other side, the International Health Organization defines the optimal amounts of calcium and magnesium which should be in the water in mg per liter, and the limits should not be exceeded to avoid intense accumulation of, for example, calcium, especially for people with health problems. Excessive amounts of salts in the water may lead to the stones formation or cause problems with the joints. Thus, achievement of the most optimal level of mineralization is an important task. Water quality is continuously monitored at the water treatment plants and the level of hardness is regulated depending on the application.

Water sources usually contain different types of water: surface and underground waters, lakes, rivers, precipitation. Depending on which amounts of natural rocks (limestone, gypsum or dolomite) were in contact with the water, the amount of calcium and magnesium may vary and thus, the value of hardness is never the same and may fluctuate depending on the area, weather, season and even amount of precipitation.

Tab water, for example, which is used for the household needs, should not be too hard, since it may lead to the problems with household appliances and pipes. At the first glance it is not possible to detect whether the water is hard or soft, but after boiling or even using it over some time period, for example, in the kitchen or bathroom, it becomes obvious: scale (a kind of crystallization) on the surfaces, or thin film on the surface of water in the kettle. This is an indication of the so-called carbonate hardness (also known as temporary). Calcium and magnesium bicarbonates under the influence of high temperatures turn into a hard, insoluble sediment – the majority of people are familiar with this process. The total hardness of water consists of both, carbonate and non-carbonate hardness and is measured in mol/m3 or mmol/l.

Not only for the human body the optimal level of hardness is of vital importance, but also for all the appliances that are in contact with water, for example, dishwashers, kettles, washing machines, pipes, boilers, heaters. Not in vain in many commercials various means are offered in order to protect the washings machines against the limescale on the working parts and to prolong its service life. Though excessive amounts of calcium and magnesium are very harmful, it does not mean that the softer the water is, the better. Tab water with very low values may also cause damage, like corrosion, to the pipes, that is why maintenance of the balance is so important.

To know the level of hardness of water is important (can be measured with a Photometer) in order to adjust the operation of the devices, to select the correct amount of lime scale cleaning agents and the system of filtration and purification in the best way. To get the detailed water analysis is possible in the laboratory, though in a daily life many people try to adjust by buying additional mediums that help to fight against scale. Application of a TDS meter is a professional approach that may be very helpful, but it provides information about all the solids dissolved in the water, so special training is recommended to use the device in the correct way.

Knowing the fact that in different countries the water hardness is measured in different units with a Photometer may significantly help to read the user manuals correctly and to adjust the household appliances produced abroad in the correct way. Thus, for example, the German carbonate hardness °DH should be converted

to °Clark in Great Britain,
to °F in France,
to ppm in the USA and
to °Ж in Russia.

It should be noted, that the values do not correspond 1 to 1 and thus application of the calculator is required to read the values correctly.

1°DH - 10 mg CaO pro l
1°Clark - 10 mg CaCO3 pro 0,7 l
1°F - 10 mg CaCO3 pro l
1 ppm - 1 mg CaCO3 pro l

In each country there are various requirements and limits regarding the hardness of water (content of calciumcarbonat in it) and it should be checked with a Photometer in accordance with the existing measuring system.

Well-equipped photometers can now be used for very many parameters in the water analysis. The measurements are comparatively simple and fast. Nevertheless, the results are comparable to those of the reference methods when performed correctly. For this reason, photometers are being used for more and more applications.

Already common applications include:

• analysis of water bodies and groundwater,
• control measurements in drinking water treatment,
• testing in wastewater treatment,
• monitoring of process water,
• disinfectant controls for swimming pool water.

Many photometers for water analysis offer several measuring angles and different wavelengths for the measurements. Special measurement sequences and comparison curves are stored in the instrument for the individual water parameters. The user usually only has to select the desired parameter in the operating menu for the respective analysis. With careful handling of samples and measuring accessories, reliable and repeatable results are obtained.

For correct measurement results in the determination of constituents, the water sample should be measured once blind, i.e. before being changed by an added reagent. Then the photometer can store the light attenuation caused by the cuvette and unchanged water as the initial value in the instrument and take it into account when determining the concentration. In contrast, when determining the turbidity value of the sample in NTU, only the cuvette is measured as the initial value.

In drinking water production and monitoring, a multiparameter photometer can be used to measure, among other things, the levels of lead, nickel, nitrate, copper, iron, as well as total hardness, pH and turbidity. In the meantime, microprocessor-controlled online photometers are also used for continuous monitoring of drinking water production plants in hazardous areas.

A legionella test specially developed for photometers provides evaluable results after just one hour. In contrast, conventional legionella tests using seed cultures are only available after one week. The patented photometric method uses for the quantitative detection antibodies and magnetic particles that bind only to living Legionella.