# Exploring Calibration Target Substrates

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:

• Standard Aluminium Composite
• Honeycomb Aluminium
• Carbon Fiber
• Soda Lime Glass
• Ceramic
• Passive Thermal Polymer
• Aluminium 3D Target
• Acryllic
• Retro Reflective Substrate

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 in the white areas and 5 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 carbon fiber with matte lithography print on carbon fiber.

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 matte lithography on carbon fiber.

## 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.

## 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, 5mm thicknesses. Other thicknesses would need to be procured on order.

On carbon fiber, we laminate a lithography printed matte Mylar sheet. The gloss-60 value is 3, making this substrate and process very matte and suitable for projector calibration.

For targets over 800x600 mm we would strongly recommend using an aluminium frame to improve rigidity.

## Soda Lime Glass Matte

Glass is commonly used for its excellent flatness and transparency. It is solely available as a substrate for our lithography process which offers accuracy in the micrometer range. Max dimension: 400mm. If the application is less demanding then we would recommend our standard Alu/LDPE substrate.

On soda lime glass, we can pattern with 100nm blue toned chrome using a lithographic etch process. This provides the very highest accuracy.

We can also laminate a lithography printed matte Mylar sheet. The gloss-60 value is 3, making this substrate and process very matte and suitable for projector calibration.

## Ceramic

We offer ceramic substrates for precision lithography processed targets for the most demanding applications. Substrates are very flat and diffusely reflecting.

On ceramic, we can pattern with 100nm blue toned chrome using a lithographic etch process. This provides the very highest accuracy.

## 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.

The passive thermal polymer 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.

## Aluminium 3D targets

We manufacture 3D calibration target out of solid aluminium, andozied and engraved.

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

## Acryllic Substrate

This substrate is available in 2, 3 and 4 mm thicknesses and can be used as a lightweight alternative to float glass for backlit applications with little to no risk of breaking. 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.

## 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.

# Other substrates

Please get in contact with us and let us know! We would be happy to investigate the use of other alternative substrates  for your application.