A chromophore is a specialized molecule, or part of a larger molecule, that interacts directly with light energy. This molecular structure is responsible for a substance’s color, as it absorbs specific wavelengths of light and reflects or transmits others. This unique property of light absorption makes chromophores the fundamental targets in laser medicine. In a therapeutic context, the chromophore selectively absorbs the laser energy and converts it into a different form, driving the desired biological change. The controlled interaction between laser light and these target molecules is the basis for various medical and aesthetic treatments.
The Core Definition and Function
The chromophore’s function is rooted in its molecular structure, which typically contains conjugated double bonds or non-bonding electrons. These structures create energy gaps that correspond precisely to the energy of photons in the visible or near-infrared spectrum. When a chromophore absorbs a photon of its characteristic wavelength, the energy boosts an electron into an unstable, excited energy state.
The molecule must rapidly release this excess energy to return to its stable ground state. In laser treatments, this energy is primarily released as heat through internal conversion. The highly localized heat generated by millions of molecules absorbing energy is the mechanism by which the target tissue is thermally altered or destroyed. A successful laser procedure hinges on matching the laser’s specific wavelength to the exact absorption spectrum of the targeted chromophore.
Key Biological Targets for Laser Light
Human tissue contains several naturally occurring chromophores that medical lasers target, each absorbing light at distinct wavelengths. The three most frequently targeted biological chromophores are melanin, hemoglobin, and water. The laser wavelength chosen is determined by which molecule is the intended recipient of the light energy.
Melanin
Melanin is a pigment found primarily in the epidermis and hair follicles, making it the target for hair removal and pigmented lesion treatments. This chromophore absorbs light broadly across the visible and near-infrared spectrum (400 nm up to 1100 nm). Lasers commonly used include the Alexandrite (755 nm), Diode (810 nm), and Nd:YAG (1064 nm). Longer wavelengths allow for deeper penetration and safer treatment of darker skin types.
Hemoglobin
Hemoglobin, specifically oxyhemoglobin in red blood cells, is the primary chromophore for treating vascular lesions like spider veins and port-wine stains. Hemoglobin exhibits strong absorption peaks in the blue-green and yellow-orange range (around 418 nm, 542 nm, and 577 nm). The Pulsed Dye Laser (PDL) at 595 nm is a standard for these treatments, as this wavelength is strongly absorbed by hemoglobin while penetrating deeply enough to reach the target vessel.
Water
Water is the most abundant molecule in human tissue and acts as a universal chromophore, particularly in the mid- to far-infrared range. It is targeted by ablative lasers used for skin resurfacing or precise tissue cutting. Water has a significant absorption peak around 3000 nm, utilized by lasers like the Carbon Dioxide (CO2) or Erbium:YAG to rapidly vaporize cellular water content. This high absorption ensures the laser energy is confined to the outermost layer of tissue, allowing for controlled removal of skin layers.
The Principle of Selective Targeting
The precision of modern laser therapy is governed by the principle of Selective Photothermolysis (SP). This concept ensures the destruction of the target chromophore while sparing the surrounding tissue. SP utilizes wavelengths that are maximally absorbed by the target but poorly absorbed by the surrounding tissue, focusing energy delivery precisely where it is needed.
Wavelength alone is insufficient for selectivity; the timing of the laser pulse is also important. This timing is based on the Thermal Relaxation Time (TRT), the duration it takes for the target structure to dissipate 63% of its initial heat. To achieve selective destruction, the laser pulse duration must be shorter than the TRT of the target.
Delivering the energy in a pulse shorter than the TRT confines the heat to the target structure, such as a hair follicle or blood vessel, preventing diffusion into healthy tissue. By adjusting the laser’s pulse duration and energy level (fluence), the clinician achieves precise thermal damage to the chromophore, maximizing the therapeutic effect and minimizing collateral damage.