Final answer:
On-scalp lighteners are specially formulated chemical lighteners, often containing bleach, that are both strong enough to achieve highlift blonding and gentle enough to apply directly to the scalp.
Step-by-step explanation:
On-scalp lighteners are the type of hair lightening products that are designed to be both effective and gentle enough for application directly on the scalp. Unlike semi-permanent hair color which cannot lighten hair, these lighteners use chemical processes, typically involving some form of bleach, to remove the natural pigment from hair. This bleaching effect is permanent, which means it can achieve the highlift blonding desired by those looking to significantly lighten their hair. It's crucial for these products to be gentle since they are in contact with the sensitive skin of the scalp.