COOLENS DTCM125 Series Bi-Telecentric Machine Vision Lenses for 1/2.5" Sensors

The COOLENS DTCM125 series consists of bi-telecentric machine vision lenses for industrial cameras with sensors up to 1/2.5" and a maximum sensor diagonal of 7.2 mm. The lenses are designed for precise optical measurements and inspection applications where low distortion, high optical resolution and constant magnification are important. The bi-telecentric optical design minimizes perspective errors and maintains nearly constant image magnification, even with small variations in the object distance.

The DTCM125 lenses combine high optical resolution with a specified optical distortion of less than 0.1% and telecentricity of less than 0.1°. This makes them suitable for dimensional inspection, edge detection, position measurement and quality control of mechanical and electronic components. The telecentric design is particularly important in applications where height differences or small variations in the object position could otherwise lead to measurement errors.

The series includes 20 lenses with an object-side field of view (FOV) diagonal ranging from 26 to 300 mm and optical magnifications from 0.277x to 0.024x. The working distance ranges from 73 to 465 mm, depending on the selected model. All models are compatible with industrial C-mount cameras. The wide selection of fields of view makes the series suitable for inspecting both small precision components and larger industrial products.

Bi-telecentric optical design for precise measurements

With conventional machine vision lenses, the apparent size of an object changes when the distance between the object and the camera changes. This perspective effect can cause measurement errors when components are not positioned at exactly the same height or distance. The COOLENS DTCM125 series features a bi-telecentric optical design, with telecentricity on both the object side and the image side. This keeps optical magnification nearly constant within the telecentric range and significantly reduces perspective-related measurement errors.

Object-side telecentricity is important for dimensional measurements of products with height variations or differences in working distance. Image-side telecentricity ensures that the light rays reach the camera sensor almost perpendicular to its surface, making the system less sensitive to small changes in the distance between the lens and the sensor. With a specified telecentricity of less than 0.1°, these lenses are suitable for precise dimensional inspection, diameter and width measurements, edge positioning and applications requiring a consistent image scale.

High optical resolution and low distortion

The COOLENS DTCM125 series is designed for industrial cameras that require high-contrast imaging of fine product details and edges. Depending on the model, the specified MTF30 is greater than 190 to greater than 210 lp/mm on the image side. This value represents the spatial frequency at which the lens still transfers 30% contrast. The manufacturer positions the series for cameras up to 5 megapixels with pixel sizes from approximately 2.2 µm. When selecting a lens, it is important to verify that its optical resolution is sufficient for the camera sensor and the required measurement accuracy.

In addition to optical resolution, geometric distortion is an important factor in measurement applications. According to the catalogue, all DTCM125 models have a specified optical distortion of less than 0.1%. This helps preserve straight lines and geometric shapes more accurately than many conventional lenses. This is particularly important when pixel coordinates are converted into real-world dimensions. For applications with demanding accuracy requirements, calibration of the complete camera system is still necessary to compensate for residual optical and mechanical errors.

Lens selection based on field of view and magnification

Selecting the correct DTCM125 lens starts with the required field of view, the camera sensor size and the required object-side pixel resolution. The series offers 20 different fields of view with object-side FOV diagonals ranging from 26 to 300 mm. These values represent the diagonal of the object field, not the width or height of the captured image. With a 1/2.5" sensor measuring 5.7 × 4.28 mm, the DTCM125-26 provides a field of view of approximately 20.6 × 15.5 mm, while the DTCM125-300-AL provides approximately 237.5 × 178.3 mm.

Optical magnification ranges from 0.277x for the DTCM125-26 to 0.024x for the DTCM125-300-AL. With the same camera sensor, higher magnification results in a smaller field of view and finer object-side pixel resolution. Lower magnification allows a larger product area to be captured, but reduces the amount of object detail represented by each pixel. The required measurement accuracy, minimum defect size and available camera resolution are therefore important factors when selecting a lens. If a larger camera sensor is required, the DTCM120, DTCM118 or other DTCM series may be a better choice.

Working distance and depth of field

The working distance of the COOLENS DTCM125 series ranges from 73 mm for the DTCM125-26 to 465 mm for the DTCM125-300-AL. Working distance is the distance between the front of the lens and the object plane for which the optical system is designed. When selecting a lens, this distance must be matched to the available installation space, product dimensions and position of any additional equipment. The working distance does not increase proportionally with the field of view for every model. It is therefore important to check the specified working distance of each individual lens.

The telecentric design provides advantages when inspecting products with height variations, but the usable depth of field still depends on optical magnification, aperture and the required image sharpness. The catalogue specifies F4.2 as the optimum aperture and provides calculated depth of field values at F11. These specifications refer to different optical conditions and should not be interpreted as maximum resolution and maximum depth of field being available simultaneously. Since the aperture of these lenses cannot necessarily be adjusted after installation, the required combination of resolution and depth of field should be determined in advance and confirmed with the supplier.

Industrial design and mounting

All COOLENS DTCM125 lenses feature a standard C-mount camera interface. Mechanical dimensions and weight vary considerably between models. The smallest model, the DTCM125-26, has a length of 103.4 mm and weighs approximately 0.3 kg. The largest model, the DTCM125-300-AL, has a length of 618.5 mm and weighs approximately 46.6 kg. Especially with larger lenses, a rigid mechanical mounting structure is essential to prevent deflection, vibration and changes in optical alignment. Models with the AL designation feature an adjustable back focal length for adapting the optical configuration.

Key features

• Bi-telecentric machine vision lenses for precise industrial measurement and inspection applications

• Designed for industrial cameras with 1/2.5" sensors and a maximum sensor diagonal of 7.2 mm

• Suitable for CCD and CMOS cameras

• According to the manufacturer, suitable for cameras up to 5 megapixels, depending on pixel size and resolution requirements

• Bi-telecentric optical design with telecentricity on both the object side and image side

• Optical distortion of less than 0.1%

• Telecentricity of less than 0.1°

• High optical resolution with specified MTF30 values greater than 190 to greater than 210 lp/mm, depending on the model

• Nearly constant magnification within the telecentric range to minimize perspective-related measurement errors

• Available in 20 different lens models

• Object-side FOV diagonals from 26 to 300 mm

• Optical magnifications from 0.277x to 0.024x

• Working distances from 73 to 465 mm, depending on the model

• Catalogue-specified optimum aperture of F4.2, with separately calculated depth of field values at F11

• Models available with adjustable back focal length (AL)

• Suitable for dimensional inspection, diameter and width measurements, edge detection and precise product positioning

• Industrial housing with model-dependent dimensions and weights of approximately 0.3 to 46.6 kg

• Standard C-mount camera interface