What are the most effective methods to achieve High Dynamic Range imaging?

As the demand for high-quality imaging grows, especially in embedded vision systems, understanding the primary methods to achieve High Dynamic Range (HDR) is pertinent. HDR imaging is about capturing more detail from scenes with varying light conditions. This helps cameras see the bright and dark parts of a scene clearly.

Through this blog, you can better understand these techniques and their practical applications. You’ll also get the pros and cons of each method, offering expert insights for those implementing or considering HDR in their systems.

First, what is dynamic range?

Dynamic range refers to the total spectrum of light captured from a specific scene. It’s a measure that quantifies the spread between an image’s darkest and brightest points. An HDR scene is characterized by bright regions (potentially sunlit areas or direct light sources) and dark areas (shadows or dimly lit regions). Traditional imaging systems may struggle to capture both these extremes in a single shot, often resulting in overexposed highlights or underexposed shadows.

Key components of HDR (High Dynamic Range)

Multi-shutter speed capture

HDR technology helps capture multiple images of the same scene using varied shutter speeds. The objective is to have one image that captures the bright areas correctly (fast shutter speed), another for the medium-lit areas (medium shutter speed), and a third for the dark areas (slow shutter speed). Each image thus records a different range of light intensities.

Image combination

Once the system has captured the different exposures, HDR technology combines these images. Using algorithms, it stitches together each exposure’s best parts to create a complete image. This composite image ensures that the bright and dark areas are well-represented, mitigating overexposure or underexposure issues.

Human eye mimicking

The human eye adjusts rapidly and perceives a wide range of light intensities in a scene, making it easy to scan environments with varied lighting while simultaneously understanding its details. HDR technology emulates this capability by combining multiple exposures.

Six effective ways to achieve High Dynamic Range imaging

1) Traditional Multi-Exposure Method

How it works: The traditional multi-exposure method captures two or three exposures of the same scene. The two-exposure approach captures one image for shadows and another for highlights, while the three-exposure method adds an extra shot for mid-tones. Post-processing then combines these exposures into a single HDR image.

Pros:

• Broader dynamic range from shadows to highlights
• More post-processing flexibility due to multiple exposures
• Enhanced image quality and reduced noise

Cons:

• Misalignment issues due to movement between shots
• Reduced frame rate due to capturing multiple images at different exposure levels.
• Time-consuming due to the need for multiple exposures and post-processing
• Imaging issues when merging exposures, especially with moving objects

2) Digital Overlap (DOL)

How it works: DOL captures multiple exposures simultaneously by overlapping pixel readout times. This allows the sensor to record different brightness levels, resulting in a wider dynamic range image.

Pros:

• Reduced motion artifacts with near-simultaneous exposures
• Single-shot data capture
• Detailed representation across shadows, mid-tones, and highlights

Cons:

• Hardware limitations related to specific camera sensors
• Complexity due to overlapping readout processes
• Noise issues in low-light scenarios

3) Digital Logarithmic Overlap (DLO)

How it works: DLO capitalizes on the logarithmic curve to enable sensors to capture both high and low light data in a single frame instead of linear pixel value readings.

Pros:

• Logarithmic responses to capture detailed information
• Reduced motion artifact risks
• Adaptive to varying scene lighting conditions

Cons:

• Requirement for unique sensors and proprietary algorithms
• Specific post-processing is necessary for full dynamic range realization
• Potential banding issues in certain tonal areas

4) Sub-Pixel/Split-Pixel

How it works: The sub-pixel/split-pixel method uses On-Chip Microlenses (OCL) within a single pixel. Each pixel contains a large and small OCL for capturing dark and bright regions. Combining signals from these two sources achieves an extensive dynamic range.

Pros:

• Blackouts in dark zones and blowouts in brightly lit areas
• Good image quality across different lighting conditions
• Reduced computational complexity, memory, and power consumption

Cons:

• Potential color artifacts in high-contrast areas
• Advanced processing might introduce delays

Looking for more information on the split-pixel technique? Check out our insightful blog on everything you need to know about split-pixel HDR technology.

5) Hyperlux

onsemi’s Hyperlux is a unique 2.1 µm super-exposure pixel image sensor family geared towards HDR imaging. It ensures SNR transitions remain above 30 dB, even at high temperatures (up to +100°C junction).

• Delivers exceptional low-light performance across a wide temperature range
• Incorporates a fully integrated LED flicker adjustment mechanism
• Offers the lowest power consumption in a minimal form factor

6) ClearHDR

Sony STARVIS 2’s ClearHDR technique captures two simultaneous images: one set at a low gain for bright areas and another at a high gain for dark zones. These two images are then synthesized into a single HDR image.

• Captures moving objects without introducing artifacts
• Features an increased saturation capacity
• Adapts to brightness changes, such as transitioning from night/day or entering/exiting tunnels

e-con Systems’ STURDeCAM31, a cutting-edge 3MP GMSL2 HDR IP69K camera

For 20+ years, e-con Systems has been designing, developing and manufacturing custom and off-the-shelf camera solutions. We have created a vast portfolio of HDR cameras, providing customization support, such as ISP fine-tuning, lens gluing/alignment, etc.

STURDeCAM34 is our recently-launched 3MP HDR camera, which is also the world’s first GMSL camera with hot plug support. It addresses the imaging needs of modern embedded vision applications with a 140dB HDR capability for delivering outstanding image clarity in diverse lighting conditions. Hence, it effectively minimizes issues like LED flicker and ensures high-quality imaging in environments with significant contrast.

Built to withstand tough conditions, the STURDeCAM34 comes with an IP69K rating, offering robust protection against dust, water, and extreme weather.

e-con Systems has previously released the STURDeCAM31 – a 3MP GMSL2 HDR IP69K camera based on an ISX031 sensor and equipped with sub-pixel technology. It has an automotive-grade 3MP CMOS sensor with an inbuilt ISP for delivering superior image quality.