Hi, everyone!  Today we will be discussing cuvette path length aka light path.  This can be a little confusing and we want to make sure you have a crystal clear idea of what the cuvette light path is and all other information associated with it.  In this short article, we will go over the following:

1. What is cuvette path length?
2. How we determine the cuvette path length
3. Light path tolerances
4. Dual cuvette path lengths

Let's go!

What is Cuvette Path Length?

The light path of a cuvette is the distance between the interior walls of a cuvette where the light goes through.  On a standard spectrophotometer cuvette, the light path or path length will be the inner distance from the front window to the back window.  Let’s take a look at some Type 1 cuvettes to help us see some of the various light paths that are available.

In a basic cuvette, you can find a 1 mm path length all the way up to a 100 mm path length.  Smaller light paths are available but those are for more specialized cuvettes such as demountable cells or HPLC flow cells.

How We Determine The Cuvette Path Length

To figure out the path length of a cuvette we need a few pieces of information. 

We’ll need:

1. The outer dimensions of the cuvette
2. And the wall thickness

Here is how to calculate the cuvette path length:

(Wall Thickness x 2) – Outer Dimension = Path Length

Let’s take Type 1UV10 as our example.

Wall thickness: 1.25 mm

Outer Dimension: 12.5 mm

1.25 x 2 = 2.5 mm

12.5 – 2.5 = 10 mm

Many people forget to double the wall thickness so it’s super important to make sure you do this.  See below for a top view of a 10 mm cuvette to see how we calculate the cuvette path length.

Path Length Tolerances

As with all dimensions on cuvettes there are special tolerances for the light paths.  The industry standard for path length tolerance is ± 0.05.  So in other words, light paths can range in size from 9.95 mm to 10.05 mm. 

Cuvettes with Dual Path Lengths

Do you need a small volume path length such as 1 mm AND a standard 10 mm cuvette all rolled into one?  Good news is that we have a number of cuvettes that can be used for this exact purpose.

Take a look at the photos below to see how this works.  Basically, depending on how you put the cuvette in your spectrophotometer will determine which path length is used.  If the light beam enters the front window, then you have a 10 mm path length cuvette.  If you rotate the cuvette 90 degrees you have a 1 mm path length.  

We have a few different cuvettes with dual path lengths available such as:

• 10x1 mm,
• 10x2 mm
• 10x3 mm
• 10x4 mm
• 10x5 mm

Now you are equipped with all the information about the path lengths of cuvettes and if you have any further questions please leave us a comment below or send an e-mail to info@fireflysci.com.

Here’s to your success!

Simcha Woitovich