# Calibration Target Substrates 101

Many applications can make due with our standard Alu/LDPE composite substrate available on our store. However, we have various options and manufacturing processes to select from when it comes to special applications. Let's take a closer look at the substrates used in previous customer projects at Calib.io:

# Available Substrates

• Standard Aluminium/Low Density Polyethylene composite.
• Lambertian Substrate
• Honeycomb Aluminium
• Uni-Directional Carbon Fiber
• Glass (float glass, soda lime or quartz)
• Ceramic
• Passive Thermal Substrates
• Solid Metal (Steel, Aluminium)
• Retro Reflective Substrate
• Acryllic

In the next sections we will go over the pros and cons of each material option starting with our standard.

## Aluminium/LDPE

This is the standard substrate we offer in our webshop.

The substrate is a composite of aluminium and Low-density polyethylene (LDPE). The LDPE, which accounts for the majority of the thickness, is sandwiched between two thin sheets of aluminium (0.3 mm each). This substrate provides a nice compromise between stiffness and weight. The patterns are transferred to the composite material using a specialised precision direct UV (Ultra-Violet) printing process resulting in a ultra durable finish. As a result it is lightweight, low cost, robust and flat. It is easily cut and machinable if mechanical features are required. We can supply targets in the following thicknesses:

 Thickness [mm] Weight [kg/m$^2$] Rigidity E·I[kNcm$^2$/m] 2 2.90 345 3 3.80 865 4 4.75 1620 6 6.60 3840

In all cases we recommend the 6 mm thickness as it is most rigid and likely to maintain its flatness. The thinner the substrate is the less area we would recommend before a special frame or fixture is required to increase stiffness. In the 6mm case, anything above 800x600 mm will require our rigid frame solution so that flatness can be maintained (See example here) . Alternatively, we would recommend our thicker and lighter Honeycomb substrates ( See below )

Some notable features of this substrate are:

• Water and sea submersible, for long periods of time. Excellent candidate for underwater calibration.
• Recommended for the harshest environmental conditions, both indoor and outdoor.
• Cleanable with a wet rag or soapy water.
• Lightweight
• Dual sided prints possible.
• Optimised for visible light.

As for optical properties, the substrate offers semi-matte characteristics and has a "gloss value" of approximately 40 @ 60deg in the white areas and 5 @ 60deg in the black. For applications using strong illumination, some care needs to be taken to avoid specular reflections. If this is unavoidable, then we recommend our Lambertian substrate.

For applications in the near infrared and above (e.g 850 nm), the substrate exhibits more specular reflection and a slightly degraded contrast. In this case we would also recommend the Lambertian substrate.

## Lambertian

Our Lambertian substrate exhibits Lambertian Reflectance properties, where in the ideal case the reflected light is diffuse and no specular reflections (mirror reflections) occur. This is possible due to the substrates fine micro-structure which scatters the light in random directions in a diffused fashion. The following image shows a comparison between our lambertian substrate and our Alu/LDPE substrate:

As it can be seen from the picture, the lambertian substrate (CharuCo target) has no specular reflections and significantly outperforms the standard substrate. The Lambertian substrate has a gloss 60 value of only 3.

The substrate is in all cases laminated to a rigid backing. Our most popular backing material is aluminium/LDPE. The honeycomb, glass, thermal and solid metal substrates can also be used depending on the application requirements.

Due to the fine microstructure on the Lambertian substrate, it is quite sensitive to damage and not nearly as robust as the Alu/LDPE substrate. Therefore extra precautions and care need to be taken in order to not damage the target. We therefore only recommend this target for lab environments.

## Honeycomb Aluminium

The honeycomb substrate consists of two 1 mm thick aluminium sheets which sandwitch a honeycomb aluminium structure. This material is known for its superior stiffness to weight ratio and flatness retention which can be seen in the following table:

 Thickness [mm] Weight [kg/m$^2$] Rigidity E·I[kNcm$^2$/m] 6 4.7 7100 10 5.0 21900 15 6.7 75500 20 7.0 138900 25 7.3 221600

For large targets, the 25 mm option would be an excellent choice over the 6 mm Alu/LPDE substrate as the weight is comparable but rigidity is nearly 58x greater.

The above image shows a large >1m x 1m charuco target at 25 mm thickness without edge banding. ABS or Aluminium edge banding can be applied upon request.

## Uni-Directional Carbon Fiber

The carbon fiber substrate is an excellent choice for calibration targets as offers an excellent stiffness to weight ratio. This substrate is available in 3, 4, 5, 6 and 7 mm thicknesses. For targets over 800x600 mm we would strongly recommend using an aluminium frame to improve rigidity. It is solely available as a substrate for our lithography process which offers accuracy in the micrometer range. If the application is less demanding then we would recommend our standard Alu/LDPE substrate. The "Gloss 60" value is 3.

## Glass / Float Glass / Quartz / Soda Lime

Glass is commonly used for its excellent flatness. We can provide this substrate in various thicknesses as a backing material for our Lambertian substrate, direct UV print or lithography processed paint if micrometer precision is required. This substrate is not recommended for large targets and shipping costs will also be higher. These substrates can also be made without a flood color, making them transparent and suitable for backlit operations.

## Ceramic

We offer ceramic substrates for precision lithography processed targets for the most demanding applications. The process deposits 100nm blue toned matte chrome coating ontro a white ceramic backing, capable of obtaining accuracy in the micrometer range. Substrates are extremely flat and can be sourced in 0.635mm and 1 mm thicknesses. Due to it's thickness there is an upper bound to how large these can be. Largest 0.635 mm is 120x120 and the 190x139 mm for the 1 mm.

## Passive Thermal Substrates

For short and long wave infrafred (SWIR and LWIR) calibration we would recommend our passive thermal substrates. The core feature of the substrate is that it has high thermal capacity and low thermal conductivity making it the ideal choice for thermal camera calibration. An added benefit is that it can also be used for standard visible light cameras which is essential for multi sensor calibration.

In operation, our customers heat up the target using an external heating source, such as a heating lamp, and measure the emissivity of the radiated heat. The difference in emissivity for black and white region is significant enough such that a good contrast is obtained:

Currently, two types of substrates are available. One is polymer based and the other a composite of our Lambertian substrate and medium density fibreboard.

Polymer based: This substrate goes through our standard UV print process and is extremely robust and offers superior emissivity contrast. It is ideal for harsh indoor/outdoor conditions and may be cleaned with a wet cloth and soapy water. It comes in a 19 mm thickness to ensure its rigidity. One downside is when using constant active heating this target can be slightly reflective and if this is the case we would recommend adding our Lambertian substrate using the Polymer backing or the medium density fibreboard

Medium Density Fibreboard: This substrate we use as a backing to our Lambertian substrate. It offers slightly reduced emissivity contrast at the cost of superior optical properties due to the lambertian substrate. However, this combination is easily damaged and sensitive. It is therefore only recommended for lab settings. Available at 19mm thickness in black backing color.

## Solid Metal (Steel, Aluminium)

Various other metal substrates can be used if the applications calls for it. These substrates would be ideal if precision mechanical features are required. An example of this would be this 3D calibration target, machined out of aluminium, andozied and engraved.

Precision CNC maching and surface grinding can be performed to obtain superior tolerances.

## Retro-Reflective

We carry special retro reflective substrates which we can laminate to any backing of your choice. The black portions of the pattern are printed using our standard UV process. This substrate is suitable for applications using active illumination close to the optical axis, requiring strong contrast.

## Acryllic Substrate

This substrate is available in 2, 3 and 4 mm thicknesses and can be used as a lighterweight alternative to float glass for backlit applications. One benefit being that our particular substrate is 30% transparent which acts as a light diffuser. It may not be as rigid as e.g glass but it won't break when dropped. We exclusively use a UV print manufacturing process which makes this an affordable, robust and reliable transparent option.

# Other substrates

Don't see a substrate listed here that you would like? Please get in contact with us and let us know! We would be happy to investigate other alternative substrates  for your application.