Optical densityPhysics

How do we know that 100% of the light is transmitted? Learn more about "transmittance" in the next section of the article.

The first thing to note is that there are four main laser types: YAG, CO2, Fibre and UV. Each has its benefits — Fiber lasers are super flexible and suitable for almost any requirement, whereas UV lasers are designed to use low-power lasers to mark materials without any visible damage.

Optical densitymicrobiology

Marking using these lasers is referred to as “Cold Marking” — this is because they can mark and process materials with minimal heat damage (this is due to their incredibly high absorption rate on a variety of materials).

Have you ever noticed how some curtains let light pass through into the room more than others?🪟 If you have to choose curtains for your bedroom 🛏️, you would want those that completely keep the light out. But if you need to build an office space that gives you a nice and bright working environment, thinner curtains would be a better fit. Why are we talking about curtains? Let's find out with our OD calculator 😉

As a related concept to optical density and absorbance, percent transmittance is the amount of light transmitted by the object or substrate. It is represented by the following equation:

Light interactions — Absorbance only takes into account the light absorption, while optical density also includes other interactions like reflection and scattering.

Optical densityspectrophotometer

Absorbance — The ability of a substrate to absorb light passing through it at a specific wavelength, leading to less transmission.

Optical density, interchangeably with absorbance, has many useful applications. The main common role is quantifying different substances in a sample and defining their concentration.

As a logarithmic ratio, optical density has no units. Anyhow, the incident and transmitted light intensities must have the same units, whether they are light intensity units (cd, mW, W/m2, lx, etc.) or arbitrary units.

Just like Fiber lasers, YAG lasers (crystal lasers) are members of the solid-state laser group. Solid-state lasers use a gain medium that is a solid, rather than a liquid (such as with dye lasers or a gas in a gas laser).

Optical densityRadiology

Applications — Absorbance is used in chemical analysis and quantification in spectroscopy, while optical density is applied to lens design and microscopy. If you're interested in learning more, have a look at the section dedicated to applications!

More or less, they represent the same concept but in a different way. They both explain the interaction of light with a substrate.

Fiber lasers have no physical moving parts. Instead, the laser uses a power source (in this case an optical fiber) doped in rare elements, typically: erbium, ytterbium, neodymium, thulium, praseodymium, holmium or dysprosium (as they have a high refractive index). By using these rare earth elements, a cheaper laser pump source can be used whilst still producing a high amount of energy as light will “bounce” around the core.

The wavelength of a UV laser is around one-third (355 nm) of a standard wavelength (1064 nm). The name “UV laser marker” comes from its wavelength being in the ultraviolet portion of the light spectrum.

High-power CO2 laser marking machines are used for cutting and welding, while low-power CO2 laser marking machines are mainly used for non-metallic materials and plastics.

Optical densityof water

Optical density and absorbance are terms representing very similar concepts, if not the same one! Both explain the interaction of light passing through a substrate, although in a slightly different manner.

UV lasers work by passing a standard wavelength laser (1064 nm) through a nonlinear crystal. By doing this, the wavelength is reduced to 532 nm. This light is then passed through another crystal, reducing its wavelength to 355 nm.

One of the earliest gas lasers to be developed, CO2 lasers remain one of the most useful types of laser and are the highest-power continuous wave lasers currently available.

To the outsider looking in, laser technologies can be confusing: from the different techniques to modify a material — such as etching, marking and engraving — to the types of systems, power amplifiers and terminology, there’s a lot to understand.

🔍​​ Are you wondering about how to calculate optical density? Practice makes progress, so let's progress together with some practical examples!

The optical density calculator is created to assist you in finding the optical density 🔅 from known incident light intensity (I0) and transmitted light intensity (I). More than that, our tool will automatically calculate the absorbance and transmittance for you based on the optical density!

The light itself is bounced between two mirrors within the tube and (once it’s strong enough) is eventually redirected to a final mirror inside the laser head, which redirects the light through a focus lens to the working material.

In UV-Vis spectroscopy, the incident and transmitted light intensity of your sample is measured as follows: I0=1.5 units\rm I_0 = 1.5\ unitsI0​=1.5 units, and I=0.35 units\rm I = 0.35\ unitsI=0.35 units:

Point of measurement — Absorbance is measured at a specific wavelength, while optical density covers every aspect of light attenuation.

Have you noticed that the only difference from the optical density equation is that there is no negative sign in the log? Hence, the relationship between absorbance and optical density is negative correlation, meaning that when one value increases, the other one decreases. Thus, we can conclude that:

The most popular type of solid-state laser for metal annealing, etching and engraving, Fiber lasers are often used to create permanent, highly detailed markings at speed. High-power Fiber lasers have wavelengths of 1064 nm — but this varies depending on the power source.

Optical densityvs absorbance

An optical density of 0 means that 100% of the incident light is transmitted and either means that the material is transparent or that it has no effect on the passage of light.

Physical density is a measure of material compactness, while optical density measures the amount of light absorbed or transmitted. Physical density has units of mass per unit volume; on the other hand, optical density is dimensionless.

To create a laser cavity, a Bragg Grating is added. A Bragg Grating is a section of glass that has stripes in it where the refractive index has been changed. Each time the light crosses a boundary between one refractive index and another, a bit of it is reflected. To contain the pump laser beam, an outer sheath is used. This way, when the laser beam bounces around in the fiber, every time it crosses the core, a bit more pump light is absorbed.

This light produces heat — heat so strong that it can vaporise materials that have melting points in the hundreds of degrees Celisus.

The difference is that absorbance measures it at a specific wavelength, while optical density considers the overall interaction and takes into account other interactions like reflection, refraction, and scattering.

Compared to standard laser light and green laser light (532 nm), UV laser light has a much higher absorption rate. This means that power doesn’t have to be increased to create vivid marks.

Physicist holding a 1st class degree and a member of the Institute of Physics. Creator of the UK vaccine queue calculator, and featured in many publications, including The Sun, Daily Mail, Express, and Independent. Tenacious in researching answers to questions and has an affection for coding. Hobbies include cycling, walking, and birdwatching. You can find him on Twitter @OmniSteve. See full profile

Samples of interest are prepared into a cuvette, and analyzed with a spectrophotometer. The absorbance noted is used to calculate the concentration with the equation: A = ε × b × c.

💡 The difference in speed of light and wavelength as it travels from one medium to another can lead to refraction. Check out Omni's index of refraction calculator and Snell's law calculator for more information.

Needham Laser have a variety of laser solutions to match different budgets and requirements. If you want to learn more about our flagship N-Lase range and their applications, visit our website.

The most common laser types in the YAG category are Nd:YAG (neodymium-doped yttrium aluminium garnet) and Nd:YVO (neodymium-doped yttrium ortho-vanadate).

CO2 laser marking machines use a carbon dioxide gas mixture, with the beam produced in a sealed glass tube filled with gas (CO2). The tube is then electrified — a high voltage passes through and reacts with the gas particles, increasing their energy and, in turn, producing light.

🤔​ Going back to the curtains concept, curtains made of materials with a lower OD, like lightweight cotton, will allow more light to pass through compared to higher OD materials like blackout fabrics.

As a result of the above, Fiber lasers are incredibly stable. They generate the beam inside the fiber, so the delivery of the beam doesn’t require complicated or sensitive optics (while normal lasers do). Furthermore, as the beam is confined to a small core, they’re very precise — so precise that their beams can be focused to a small dot.

As a result, UV laser marking is ideal for applications that require high contrast or minimal product damage (it’s widely used in the medical industry for marking on pill bottles and medical devices, for example).

Optical densityunit

With an environmental mind, Kat is currently pursuing a Master’s degree in Biomass Engineering and Bioeconomy. Kat joined Omni as an Italian and French translator. Apart from translating, writing content, and sometimes creating calculators, she loves nature, reading, sports, and playing lots of board games. See full profile

These lasers have the same wavelength as high-power Fiber lasers — 1064 nm — making them suitable for marking metals and plastics. However, unlike Fiber lasers, these laser types use expensive and unreliable pump mechanisms: bulbs. These pumping mechanisms break easily and have a relatively short lifespan compared to other laser systems.

Optical density (OD), also known as optical absorbance, is the value indicating a material's property according to the speed of light passing through an optically dense medium. The mechanism that makes an object optically dense is its ability to maintain the electromagnetic waves in the form of electron vibrations before reemission into another medium.

Optical density is dimensionless and has no units. Since it is a representation of the attenuation of light as it passes through a medium, there are no units associated with it. Otherwise, we can think of optical density units as "arbitrary units" as well.

Since they are very similar, the two terms can be confusing. Don't worry! The optical density calculator has got your back 😉​

Optical density (OD) is the value indicating the ability of an optically dense object to maintain or delay the speed of light emitted through it in the form of electron vibrations before reemission into another medium.

Microbiology — Quantifying bacterial growth via the analysis of the diluted microbial culture into the spectrophotometer.

Also known as a flash lamp or lamp-pumped laser, YAG lasers utilise a lamp (bulb) as a pumping mechanism and a crystal as the gain medium. Both reside in an optical resonator which reflects the light and helps create the laser light.

However, it’s not all that complex. What makes laser technology seem difficult to grasp is the sheer amount of information available and knowing where to begin.That’s why, in this blog, we break down this information to give you a brief overview of how different types of lasers work and what their benefits are.

These machines can achieve marking speeds of up to 2000 characters per second (with a 6mm laser head) and are much smaller than CO2 lasers. They’re also much more energy efficient and powerful: they convert around 70–80% of the energy they use and produce a laser beam that’s 100 times more intense than a CO2 system.

Optical densityformula

What if the incident and transmitted light intensity are the same value? Let's say that I0=5 units\rm I_0 = 5\ unitsI0​=5 units and I=5 units\rm I = 5\ unitsI=5 units:

Environmental monitoring — Measuring the contamination or concentration in soil, wastewater, or samples from water bodies.

While measuring the concentration of cells in your culture, the spectrophotometer measured I0=0.80 units\rm I_0 = 0.80\ unitsI0​=0.80 units and I=0.10 units\rm I = 0.10\ unitsI=0.10 units:

Does it look familiar? 🤔​ Exactly, the percent transmittance, or the ratio between incident and transmitted light intensity, is used to calculate both optical density and absorbance!