smartPhotoMeter, smartColoriMeter and smartSpectroMeter

Easy-to-use OpenSource devices for measurement of transmission, emission and absorption of light at given ranges of wavelengths of colored or not colored substances in solution.

• For analytics of aqueous solutions.
• Works with ultraviolet, visible and near infrared light ranges, by wavelengths around 320 up to 950 nm.

Allows CIE compliant measurements of colors and their transformation into various color spaces and color systems including the determination of colour differences to common color indices/references according to classic as well as modern standards (ΔE CIE 1976, ΔE CIE 1994, ΔE CIE 2000, ΔE CMC(1:1), ΔE CMC(2:1), ΔE DIN99).

Turbidity – a source of error
The turbidity of a sample is often underestimated, because often it is not possible to see it visually.
The nephelometric turbidity of a sample is measured and displayed for every single measurement automatically.

Due to their wide range of application, their reliability and robustness

	Transmission	Emission	Absorbance	Colorimetric analysis	Spectral analysis	Nephelometric turbidity
smartPhotoMeter	X		X			X
smartTurbidiMeter	X		X			X
smartColoriMeter				X
smartSpectroMeter	X	X	X	X	X	X

they can be perfectly used as portable analytical instruments for outdoor analytics.

Your insurance for secure measurement results.

Electromagnetic radiation used for photometric and spectrum analysis

In simple terms, photometry is a measurement method for determining the concentration of colored solutions with the aid of electromagnetic radiation (“light”).

The two most common types of photometers are:

• Spectrophotometer
• Filter photometer

In spectrophotometers a monochromator (with prism or with grating) is used to obtain monochromatic light of defined wavelengths. They can also be used to scan the spectrum of the absorbing substance. Spectrum scans can be used for additional colorimetric analysis at a wavelength range from 360 nm to 830 nm.

In filter photometers, optical filters are used to give the monochromatic light.

smartPhotoMeter

Due to the evolution and availability of various types of LEDs it is easily possible to obtain nearly monochromatic light with specific wavelength ranges for respective measurements, but without the need to setup an complex and sensitive monochromator or filter mechanismn.

• The selection of the optimum wavelength range depends on the color of the sample solution and the associated absorption of the complementary color.
• For a blue solution, a wavelength range near to yellow should be used (green LED), while for a yellow solution a blue LED should be used, since always the complementary color is absorbed.
• The more closely the absorption maximum of the substance to be detected is approached, the higher the performance of the photometer will be.
• The smaller the spectral bandwidth of the LED (the range between lower and upper wavelength), the higher the photometer resolution.

smartTurbidiMeter

The photometric determination of turbidity at a wavelength of 850 nm only to circumvent additional absorption of color in the visible region (following DIN EN ISO 7027).

• Double-beam measurement by using two independant IR light sources and optical paths

smartColoriMeter

An RGB sensor with color sensing photodiodes (red, green, blue) can be used for colorimetric analysis.

An IR blocking filter, integrated on-chip and localized to the color sensing photodiodes, minimizes the IR spectral component of the incoming light and allows color measurements to be made accurately.

smartSpectroMeter

Due to the evolution and availability of various types of spectral sensors it is easily possible to measure the intensity of light at a specific wavelength range, but without the need to setup an complex and sensitive monochromator or filter mechanismn.

By using a low-voltage quartz halogen capsule lamp (Quartz-UV Open) and an array of spectral sensors a spectrum for the range of UVA, VIS and IR-A light can be obtained accurately.

These spectral sensors have 20 independent on-device optical filters whose spectral response is defined in a range from 330 nm to 940 nm with a Full Width Half Max (FWHM) of 20 nm.

Spectrum scans can be used for qualitative, quantitative and colorimetric analysis at a wavelength range from 360 nm to 830 nm.

Cuvette

The selection of an appropriate cuvette for each measurement is crucial. The material (depending on the wavelength) and size of the cuvette are the two key factors.

• High-quality materials such as quartz are used in rectangular standard cuvettes for measurements in the UV range.
• In addition to round and rectangular cuvettes, there are also special cuvettes, such as flow-through cuvettes, where the sample solution is passed into and out of the cuvette by a pump.
• The size of the cuvette is directly related to the layer thickness that is effective in the Lambert-Beer law. Generally speaking, the larger the cuvette and the sample volume, the greater the layer thickness, and the more sensitive the measurement can be.

Setup options

Ultraviolet radiation
	100-280 nm	UVC		UV transmitting quartz glass cuvette > 190 nm	Numerous anorganic and especially organic compounds absorb UV light
	280-315 nm	UVB
	315-400 nm	UVA		Borosilicate glass cuvette > 290 nm
Colors of light and complementary colors
	400–435 nm	Violet	Yellow-Green	Normal glass (soda-lime glass) cuvette > 350 nm	Anorganic compounds by using calibrated test
	435–495 nm	Blue	Yellow
	495–520 nm	Cyan (Blue-Green)	Red
	520–570 nm	Green	Violet
	570–600 nm	Yellow	Blue
	600–625 nm	Orange	Cyan (Blue-Green)
	625–700 nm	Red	Cyan (Blue-Green)
	700–780 nm	Deep Red	Cyan (Blue-Green)
Infrared radiation
	780-1400 nm	IR-A (NIR)			Nephelometric turbidity measurement Biological samples Sectroscopic analysis of aqueous solutions
	1400-3000 nm	IR-B (SWIR)
	3000 nm - 1 mm	IR-C (MWIR, LWIR, FIR)

Reducing complexity

• by using low-voltage RGB, UVA and IR-A LEDs and quartz halogen capsule lamps (Quartz-UV Open).
• by the integration of various digital light and spectral sensors orchestrated by simple microcontrollers.
• by distributing different analysis methods on multiple meshed devices.
• by the strict segregation of data acquisition, processing, transmission, analytics and their visualisation on different layers and devices.

Being Smart - The difference

Connect all devices (Microcontroller including their connected LED/Halogen fireworks and light sensors) via WiFi to your internal network and orchestrate them by using simple web- or MQTT-based APIs.

Any device (e.g. PC, Laptop, Tablet, smartPhone, ...) with a standard web browser can be used to access a simple web page on the microcontroller (MC).

A Javascript application will start within your browser and is triggering and managing all analytis, evaluation of measured values and their visualisation.

Environmental monitoring

Monitor and secure your environment in real time with online monitoring by logging readings into a central database with MQTT-based APIs.

Absorbance measurement

The principle of absorption photometers is that (as far as possible) monochromatic light is allowed to pass through a cuvette containing the solution. It then reaches a light sensor, that measures the intensity of the light compared to the intensity after passing through an identical cuvette with the same solvent but without the colored substance. From the ratio between the light intensities, knowing the capacity of the colored substance to absorb light, it is possible to calculate the concentration of the substance using Lambert-Beer's law.

λ	Wavelength [nm]
Bo	Beam (measuring) angle [o] 180 o: Transmitted light measurement (Absorbance) 90 o: Nephelometric turbidity (scattered light) measurement
I0	Intensity of incident light [lux]
IT	Intensity of transmitted light [lux]
T = IT / I0	Transmission Energy passing through Range: 0.0 - 1.0
A = 1 - T	Absorption Energy lost
Eλ = - log ( T ) = log ( 1 / T ) = ελ * c * d	Absorbance Attenuation Lambert-Beer law Optimum range: 0.1 – 1.0
SACλ = Eλ / d * 100	Spectral Absorption Coefficient [m-1]
c = Eλ / ( ελ * d ) = Eλ * F = Eλ / m	Concentration of absorbing substance in the liquid [mol/l]
F = 1 / m = 1 / ( ελ * d )	Factor Test specific
m = ΔE / Δc = ελ * d	Slope
N	Zero value
ελ	Molar decadic extinction coefficient [l/mol/cm]
d	Thickness of the solution layer [cm]

NTU check

• Safely spot interfering turbidities
• Unique for maximum measurement result safety
• Runs automatically with each measurement

Color measurement

Color assessment focusses on the optical characteristics of the material, i.e. its ability to modify incident light waves. If an object is exposed to light, it reflects a certain portion of the light, absorbs another portion and transmits the rest.

• Reflectance is the basic value for color measurement at reflecting materials (surfaces).
• Transmittance is the basic value for color measurement at transparent materials (clear liquids, foils).

Color perception is, like any other spatial perception, three-dimensional. This means that colors can be described by three clear measures of quantity like e.g. lightness, hue and saturation.

CIE 1931 is a Color Matching System that specify a measured color numerically by 3 functions called the RGB color matching functions.

Color Matching functions have three fixed primary colors and each function outputs the amount of primary needed to create a desired color when all three are mixed. 700nm, 546.1nm and 435.8nm are the primaries of the 1931 RGB color matching functions: r(λ), g(λ), b(λ). Plotted below.

Die Werte X, Y und Z werden Normfarbwerte oder auch Tristimulus-Werte genannt.

A colorimeter is generally any tool that characterizes color samples to provide an objective measure of color characteristics.

Color parameters:

• Conversions of measured color into various color spaces
• Measuring of color distance/difference against quality control standards (e.g. color scales/indices/references)

Color Spaces

Normally, color spaces are three dimensional spaces with a color component on every axis. The colour of an object is defined by combining these three elements.

The following color spaces are supported:

Φ	Luminous flux [lm]
Ω	Solid angel in Steradian [sr]
E = Φ / A	Illumination [lux] [lm / m2]
I = Φ / Ω	Luminous intensity or candelpower [mcd] [lm / sr]
	Clear (unfiltered) value
RGB are tristimulus values r = R/(R+G+B) g = G/(R+G+B) b = B/(R+G+B) rgb are chromaticity coordinates and r + g + b = 1	R, G and B (filtered) values
X = a * Tx + b * Tz Y = Ty = Illuminance Z = c * Tz a, b and c depend on illuminant and observer Standard illuminant C and 2°-observer	Standard tristimulus values CIE-1931 color space (tristimulus system)
x = X / ( X + Y + Z ) y = Y / ( X + Y + Z ) z = Z / ( X + Y + Z ) x + y + z = 1	Chromaticity coordinates in the tristimulus system
TC = 449 * n3 + 3525 * n2 + 6823.3 * n + 5520.33 where n = ( x − 0.3320 ) / ( 0.1858 − y )	Light color temperature (CCT) in Kelvin [oK] McCamy’s formula
L* = 116 * ∛( Y / Yn ) - 16 a* = 500 * ( ∛( X / Xn ) - ∛( Y / Yn ) ) b* = 200 * ( ∛( Y / Yn ) - ∛( Z / Zn ) ) C* = √( a*2 + b*2 ) hab = arctan( b* / a* ) Xn, Yn and Zn depend on illuminant and observer	CIE-L*a*b* colorimetric system
Calculated from standard tristimulus values X, Y and Z, but with different equations The color space is related to the CIE-Lab space in purpose, but differs in implementation	Hunter-Lab-values Used for the assessment of surface colors

Color Distance

To determine color differences according to classic as well as modern standards (ΔE CIE 1976, ΔE CIE 1994, ΔE CIE 2000, ΔE CMC(1:1), ΔE CMC(2:1), ΔE DIN99).

ΔE* ΔE*wp: Color difference between the sample and water ΔE*ws: Color difference between the standard solution and water ΔE*wp < ΔE*ws

CIE-L*a*b* difference values

Color Scales

Color scales/indices/references are common and simple methods to determine the color of a sample.

The sample is compared with a series of colored standards/references. The result corresponds to the standard with the most similar colors.

Color scale	Range	Calibration / Standard	Measurement
Iodine Color Number	0 - 120	mg of iodine per 100ml potassium iodide solution (DIN 6162)
Hazen Color Number (APHA)	0 - 1000	Measure yellow hues in liquids (DIN ISO 6271) Calibration standard with 500 mg/l Pt/Co 500 (500 Hazen)	E430 = 0.115 E455 = 0.135 E480 = 0.115 E510 = 0.06 (Optical path length: 10 mm)
Gardner Color Number	0 - 18
Lovibond Color System	Lovibond® 1) yellow/red Ly 0 - 120, Lr 0 - 20
European Pharmacopoeia (EP) Color Determination	B, BY, Y, GY, R	Colors in CIE-L*a*b* colorimetric system
US Pharmacopoeia (USP) Color Determination	A - T	Colors in CIE-L*a*b* colorimetric system
Chinese Pharmacopoeia (CP) Color Determination	OR,OY,Y,YG,BR	Colors in CIE-L*a*b* colorimetric system
Klett Color Number	0 - 1000	Absorption of a sample liquid in a square cuvette of 4cm (or 2cm) path length measured through a blue filter	KlettPhot = E470
Hess-Ives Color Number		Cosmetic industry, color evaluation of fat derivatives R, G and B are the color components for the red (640 nm), green (560nm) and blue (464nm) shares	H-I = (R + G + B) * 6 / layerthickness R = 43,45 * E640 G = 162,38 * E560 B = 22,89 * ( E460 + E470 ) / 2
Yellowness Index (ASTM D1925)		For transparent liquids on the basis of CIE XYZ-tristimulus values, standard illuminant C and the 2°-standard observer.	Yi = 100 * ( Tx - Tz / Ty )
ADMI Color Number	0 - 500	Colors in CIE-L*a*b* colorimetric system
Acid Wash Color Determination	1 - 14	Colors in CIE-L*a*b* colorimetric system
EBC Brewery Color Number		Optical path length: 10 mm	25 * E430
ASBC Brewery Color Number		1 ASBC = 0.375 EBC color + 0.46	12.7 * E430
SAC436		Spectral Absorption Coefficient describes the yellow colouring of potable-, used- or wastewater. The measuring range is indicated in m-1. (Extinction per meter optical path lenght [Ext/m]).	E436

European Pharmacopoeia (EP) color scales in CIE-Lab* colorimetric system (C°2, d=5cm)

US Pharmacopoeia (USP) color scales in CIE-Lab* colorimetric system (C°2, d=5cm)

Chinese Pharmacopoeia (CP) color scales in CIE-Lab* colorimetric system (C°2, d=5cm)

Acid Wash color scales in CIE-Lab* colorimetric system (C°2, d=5cm)

Comparison of ASTM D1500 and ASTM D156 (Saybolt) color

Comparison of visual color systems with the Z-transmittances (C°2, d=1cm)

Iodine color number (C°2, d=1cm)

Hazen number (C°2, measured with d=5cm, calculated to d=1cm)

Gardner color scale (C°2, d=1cm)

Iodine, Hazen and Gardner color scales in CIE-Lab* colorimetric system (C°2, d=1cm)

ADMI and Hazen color scales in CIE-Lab* colorimetric system (C°2, d=1cm)

Turbidity measurement

Turbidity (opacity, clouding, haze) is an optical property of a liquid sample describing the degree of clarity. Turbidity is caused by small suspended (insoluble) particles having a refractive index different from the medium. This results in absorption, scattering and reflection of the incident light. In general, the higher the turbidity, the more intense is the resulting scattered light. Turbidity particles absorb at virtually all frequencies, due to their three-dimensional structure.

Additional absorption of color in the visible region may be circumvented by a measurement in the infrared (IR) or near-infrared (NIR) range. Accordingly, the attenuation of transmission is a measure of the concentration of turbidity-causing solids. The photometric determination of turbidity is usually done at a wavelength of 860 nm (DIN EN ISO 7027).

Factors that influence this intensity:

• Wavelength of the incident light
• Particle size and shape
• Refractive index and color of the sample

Photometry is an objective measurement that allows the comparison of different levels of turbidity:

• Double-beam measurement by using two independant IR light sources and optical paths

  • In case of heavy turbidity
    • Absorbance measurement (transmitted light measurement) at a 180° beam angle from 2-400 NTU

  Recommended measuring range: 40–4000 FAU (DIN EN ISO 7027) Light source and detector are located on the same axis (180° angle). The reduced light intensity which remains after passing through the sample is measured.

  • In case of low turbidity
    • Nephelometric stray light measurement at a 90° beam angle from 0.1-1000 NTU The intensity of the light which is scattered by undissolved particles in the sample is measured. The international unit is the nephelometric turbidity unit, NTU. Other units are FTU (formazine turbidity unit), FNU (formazine nephelometric unit) and TU/F (turbidity unit formazine). Recommended measuring range: 0–40 FNU (DIN EN ISO 7027) The international unit is FAU (formazine attenuation unit). In Germany, the spectral absorption coefficient SAK [1/m] is often used.

· Transmitted light measurement at medium to high turbidity concentration (about 40–4000 FAU); used e.g. for determination of the solid portion in activated sludge. · Light scattering measurement at low turbidity concentration (about 1–40 FNU); e.g. in treated wastewater.

λ	Wavelength [nm]
Bo	Beam (measuring) angle [o] 180 o: Transmitted light measurement (Absorbance) 90 o: Nephelometric turbidity (scattered light) measurement
I0	Intensity of incident light [lux]
IT	Intensity of transmitted light [lux]
T = IT / I0	Transmission Energy passing through Range: 0.0 - 1.0
A = 1 - T	Absorption Energy lost
Eλ = - log ( T ) = ελ * c * d	Absorbance Attenuation Lambert-Beer law Optimum range: 0.1 – 1.0
SACλ = Eλ / d * 100	Spectral Absorption Coefficient [m-1]
c = Eλ / ( ελ * d ) = Eλ * F	Concentration of absorbing substance in the liquid [mol/l]
F = 1 / ( ελ * d )	Factor Test specific
ελ	Molar decadic extinction coefficient [l/mol/cm]
d	Thickness of the solution layer [cm]
Turbidity	Opacity, clouding, haze
FNU	Formazine Nephelometric Unit
NTU	Nephelometric Turbidity Unit
FAU	Formazine Attenuation Unit

smartPhotometer are in this way cheap, robust and easy-to-use, but very flexible and also provide multiple analysis and reporting options which normally are only available in very expensive high end solutions.

Don't reinvent the wheel and rely on agile open source communities.

Tips and tricks

• The filling level of the cuvette must be at least 4 cm due to the beam path of the photometers.
• Turbid samples have to be filtered; turbidity leads to incorrect results.

Datasheet

Measuring modes		UV SAC	Color	VIS SAC / Turbidity				Turbidity		Spectrum SAC / Turbidity
	Target wavelength	254 nm	VIS	VIS, 436 nm			850 nm	850 nm	850 nm	UVA, VIS, 436 nm, NIR	850 nm
	100-400 nm	UV								UVA
	400-780 nm		VIS	VIS						VIS
	> 780 nm						IR	IR	IR	NIR	IR
Device		smart-UV-Photometer	smartColoriMeter	smart-PhotoMeter				smartTurbidiMeter		smartSpectroMeter
	Measurement method	Absorbance	Color index	Absorbance			Nephelometric turbidity	Absorbance	Nephelometric turbidity	Absorbance, Color index	Nephelometric turbidity
	Beam angle [o]	180	180	180			90	180	90	180	90
	LED(s) and Sensor chip mounting height [mm]	20.0
Readings
	Illuminance (EV) [lux = lm/m2]	X		X	X	X	X	X	X	X	X
	Color index		RGB
	Light color temperature (CCT) [oK]		X
LED(s)		NSPU510CS	504WC2E-W6-3PC	LL-509RGBC2E-006			HIR383C-L289	2 x HIR383C-L289		7387 10W G4 6V 1CT/10X10F	HIR383C-L289
	Light color	UVA	Warm White	Red	Green	Blue	IR	IR	IR	Warm White	IR
	Color Rendering Index (CRI) [Ra]		80							100
	Light color temperature (CCT) [oK]		3000							3200
	Standard illuminant (CIE 1931/2o)		LED-B2							Halogen
	Chromaticity Coordinates (CIE 1931/2o)		x = 0.43 y = 0.40
	Wavelength (λP) [nm]	375	410-780	626	525	470	850	850	850
	Spectrum half width (Δλ) [nm]	9		20	40	26	45	45	45
	Luminous intensity IV [mcd] (by I = 12 mA)		13800	1920	2400	1200
	Luminous flux (ΦV) [lm]			1.0	1.2	0.6
	Viewing angle (2θ) [o]	15	40	25			18	18
	Ø [mm]	5
	Length [mm]	8.3	8.65	8.6			8.6	8.6
	Long leg	Anode (+)	Anode (+)	Common Cathode (-)			Anode (+)	Anode (+)
	IF [mA]	20	25	35			100	100
	VR [V]	4	5	5			5	5
	VF [V] (by I = 12 mA)	3.4	2.9	1.93	2.9	3.4	1.3	1.3
	Calculated series resistor [Ω]	0	33.3	114.2	33.3	0	175	175
	Series resistor [Ω]	0	62	182	62	0	182	182
	GPIO	12	12	12	13	14	23	12	23
Cuvette		UV quartz cuvette	UV quartz cuvette	Normal glass (soda-lime glass)				Normal glass (soda-lime glass)
	Material	UV transmitting quartz	UV transmitting quartz	Normal glass (soda-lime glass)				Normal glass (soda-lime glass)
	Chemical and heat resistant	Yes
	Transmission range [nm]	190-2500	190-2500	350-2500				350-2500
	Form	Square	Square	Round				Round
	Polished sidewalls	2	2	N/A				N/A
	L x W x H [mm]	12.5 x 12.5 x 45.0	12.5 x 12.5 x 45.0	N/A				N/A
	Ø x H [mm]	N/A	N/A	16.0 x 200.0				16.0 x 200.0
	Wall thickness [mm]	1.25	1.25	1.00				1.00
	Light path length [mm]	10.0	10.0	14.0				14.0
	Sample volume [ml]	3.5	3.5	>5				>5
Light sensor		VEML6070	TCS34725	TSL2591				TSL2591
	Wavelength (λP) [nm]	355	525	650			850	850
	Spectrum half width (Δλ) [nm]	20	125	150			42	42
	Spectrum range (λR) [nm]	320-380	380-780	380-780			780-910	780-910
	Voltage [V]	3.3-5.0
	I2C GPIO	SCL: 5 SDA: 4
	I2C address	0x10	0x29	0x29				0x29
	L x W x H [mm]	13.0 x 12.0 x 2.0	20.3 x 20.3 x 2.0	19.0 x 16.5 x 2.0				19.0 x 16.5 x 2.0
Microcontroller		ESP32-WROOM-32 NodeMCU WLAN/WiFi
	L x W x H [mm]	48.3 x 25.4 x 10.0
	Firmware	ESP_Easy_mega_20201102_test_ESP32_4M316k-factory.bin
	Update	Via Web-GUI by using OTA (Over the Air)
Case		OpenSCAD 3D model (smartPhotometer-3d-model.scad)
	3D printing	via STL exports
	Filament	Black PLA
	Layer height [mm]	0.2
	STL exports	smart-UV-Photometer-Case.stl	smart-Colorimeter-Case.stl	smart-RGB-Photometer-Case.stl				smart-IR-Turbidimeter-Case.stl
		smartPhotometer-Case-Lid.stl
		smartPhotometer-Cover-for-square-Cuvette.stl
		smart-UV-Photometer-Cuvette-Holder.stl	smart-Colorimeter-Cuvette-Holder.stl	smart-RGB-Photometer-Cuvette-Holder.stl				smart-IR-Turbidimeter-Cuvette-Holder.stl

Sources

• David Harvey (DePauw University) "Analytical Chemistry 2.1", Chemistry LibreTexts (TM) 2020, 10.
• Bruce Justin Lindbloom: http://www.brucelindbloom.com/
• Chandler Abraham "A Beginner’s Guide to (CIE) Colorimetry", 2016

smartPhotometer by smartAquaMetering is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
Based on a work at https://github.com/smartaquametering/smartPhotometer.
For permissions beyond the scope of this license see https://github.com/smartaquametering/smartPhotometer/blob/main/LICENSE.